That's a very good question. The most common answer in this and other forums seems to be Windows, games, software, and other applications will initially load faster. In addition a few items that are called up will also load faster. For example, if you are playing a game and need to consult a map, the map might load faster. The improved speed and performance can vary quite a bit between different types of ssd's and applications.
The downside is the price. For the time being SSD's are expensive for consumers. It will be a few years before prices are reduced significantly. In addition, ssd's will not improve anything done on the Internet.
There is also the issue of "bragging rights" - be the first one with the newest, fastest, biggest, beast available.
My desktop computers have fast hard drives and boot pretty quickly, but I have some apps to run that do take a long time to load. An SSD would help with this. My home desktop computer doesn't run any big apps so I don't see that there would be any benefit for me to put an SSD in that one.
Also I have 2 laptops and they take a long time to boot, probably due to relatively slow hard drives. I would like to convert both of them to SSD. This would help them to boot faster, and make them hybernate and wake up faster too.