Why do you switch from winter to summer tires in the spring? I'm new to car ownership

and I don't know the rational. Is it because winter tires use up more gas?

I haven't switched tires in years...all weather tires..

It costs the state more in pavement repairs so they have an interest in keeping the costs down. Perhaps they have the same rules where you live so you might check the dates out as a responsible driver.

My wife's Honda weighs like four pounds. It simply needs the snow tires to get around in winter. The only reason I don't have them for my Jetta is because individually they are kind of pricey and these days tire shops insist on selling four rather than the necessary two. The reason, they insist, is because front drive cars need four despite the fact that steering, drive and most of the braking is all done on one axel.

Lower rolling resistance (there's the gas savings), less wear (cost) per mile, better traction, better tracking (easier to drive, requiring fewer steering corrections). Also, the "summers" are usually quieter, and often ride better.
If you use the European approach ( Winter and summer tires each mounted on their own set of wheels), tire profiles/sizes can be optomized as well for the respective operating conditions. The cost and inconvenience of seasonal changeover is considerably less, as well - typically paying back the cost of the extra wheels in a couple years.

Summer tires are useless in the snow and winter tires wear quickly and perform poorly in dry conditions. All season tires are a decent compromise but do not perform as well as either summer or snow tires in their element.

Not that any of us ever drive fast.

Some true snow tires carry a lower (Q) speed rating (99 mph). This is still above the USDOT 85 mph standard - but lower than the (S) speed rating customary on most import tires. Most folks are fine with this.

The bigger problem comes with high-performance tires, typically bearing H, V, or Z speed ratings, and NOT having the sidewall "M&S" (mud 'n snow) designation. These tires are likely to have a tread compound which enters a "glass state" below 32F/0C - and become brick hard and lose much of their traction as a result.

Here's a link with a far more detailed explanation of speed and load ratings:
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35

With the drive in the rear wheels that had little weight on them and there fore little bite in snow, snow tires with deep tread and soft er body could get much more traction in the snow.
Two cars did not need snow tires back then - the VW and the Corvair - because the weight was over the rear wheels.
When the drive train was moved to the front wheels and the weight of the engine was over the drive train, the need for snow tires dramatically decreased.
I live in Iowa and haven't had a need for snow tires for decades. Just some tires with good treads.
Plus, the highway department does a much better job of clearing roads with their pretreatments and all.

There are still a few rear wheel drive cars i think. And of course pick ups are rear wheel drive.

My wife has them on her Civic Hybrid and they make a big difference. Many cars in the 70s and earlier were so big that their rear sections (which included a big differential) were nearly as heavy as the front end of a recent car. OF course, it had more to push too which may be where much of the front drive advantage comes from.

There are still a few rear drive cars. They tend to be pricey sports cars since rear drive affords better weight distribution and is not as prone to steering biases.

But the front wheel drive has major advantages over the cars of yore. I have felt no need to buy snows since I got my first FWD 20 + years ago.
And at least where I live, snow removal is so much, much better than even 10 years ago.

And there have been VW's in our (NH)family since 1956. Coupla reasons - even with weight bias over the drive wheels, you need significant tread and lots of edges to gain forward traction on snow, and more so on slush, wet snow, or mud. And you need to steer and stop as well, which requires a fairly square, treaded shoulder.
If anything, you need snows on a front-drive even more, because as soon as they start to spin, you lose steering as well, and vise versa.
Common radials are better than nothing (and are responsible for the reduction in demand in snow tires) - but it should be noted that they are no longer rated all season below 1/2 tread, and you better stick to the main roads...

certainly better to bet on the side of safety.

In Texas (or probably most of the south and the west coast) I wouldn't need winter tires. But if you live where it gets very cold or snowy, I guess snow tires are a good idea.

They give a rough ride, make noise and detract from fuel economy. Plus they do not stick to the road as well as regular or all-seasons do. Also, their knobby tread wears out pretty quickly on hard pavement. Snow tires sacrifice those things in favor of not getting stuck or losing control in a storm.

Like a Nokian Nordman - the compromise is not nearly as bad - and they will survive conditions (3/4 ton, x-cab 4x4, fierce driver) that kill domestic all-seasons.

Traction in Snow and Ice was superb.

but they are like having teeth on your wheels in the summer.....

The soft water sipping compound used for Ice Traction wears quickly on hot pavement. That and the deep snow lugs make more noise and generally don't corner as well as a summer tire.

need every bit of traction possible

I know a couple old racers who call tires "sneakers", and it's a very useful metaphor.