You need encryption for the same reason you need locks. Locks are not the only trick to provide you security but they are pretty much a very necessary tool. Without solid encryption you can't have a lot of the good things on the internet: online banking, online shopping, filling taxes online,...
> it is still pretty easy to create a virus or something that access the data at the moment it is used
It is not so much anymore. Viruses are getting quite harder to make, even for Microsoft Windows. And, even if you were right, you still need encryption for people that are smart enough to avoid viruses.