From what I hear and read it lets you store 100% of Apps on SD instead of partial apps, higher class card runs faster, lets your phone run faster and smoother.
From Android Forums:
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..the issue is that the evo has a small amount of ram on the phone. not sure what htc was thinking when designing the evo. but there is the apps2sd that can be sort of helpful. when you go to settings/applications/manage apps and then move the apps to the sd card, you are using apps2sd. it moves the apps to the sd card, but it only moves part of the app to the card. the other part is still left on the phone. that is why you can't use any widgets attached to the app when it is moved to the card.
so what is taking up all of the ram then? well your phone, as it is, has the dalvik cache which the phone uses to load any executable files that it needs. and then there is cache where your temp files are stored. of course there is your apps. and finally there is the stock rom with sense 1.0. so you can see how fast your ram will disappear.
the only way to get around this is to be rooted. the custom roms that are out now are much smaller and faster then the stock rom. plus you can partition your card and use a2sd which is different then apps2sd. a2sd moves the entire app to your card. the only thing left on your phone is a symbolic link that tells the phone where to find the app on your card. it saves a ton of ram by doing this. just to give you an idea i have 160 apps with still 250mb left!
http://androidforums.com/evo-4g-all-...d-edition.html