Deleting branches after merging them into other branches can help unclutter your list of branches. If you use a lot of topic branches for features and don't delete them, pretty soon you'll have a very long list of old, unmaintained branches, and that's going to make it harder for you to find the ones you're currently working on.
You can always get back to old branches anyway. When you merge, a merge commit is created on the branch you merged into. Merge commits have two parents, which are referenced by their SHA1s. So if you need to see the old branch, just check out the SHA1 of the corresponding parent on the merge commit. Since it's referenced in a living branch, it's not gonna get garbage-collected by Git.
The only case where the original SHA1 wouldn't be kept around is if it was a fast-forward merge. To prevent that from happening, you can use the option --no-ff on the git merge command, which will create a merge commit even when a fast forward would be possible.