I couldn’t say it better than Gizmodo does:

But honestly, the fancier Google Photos is far superior anyway with its unlimited photo and video storage and image recognition tech that makes searching your photo library a cinch. If you’re someone who’s pumped Picasa full of pics and videos, you can access that content in Google Photos.

Picasa isn’t going to stop working, but it isn’t going to get any more updates. Check out the team blog if you’d like more details.