This simplifies backporting of packages.
For example, stretch-backports has debhelper 12~bpo9+1.
This version will match "debhelper (>= 12~)" condition
and will not match "debhelper (>= 12)".
From debian-policy 5.6.12. Version [2] :
The lexical comparison is a comparison of ASCII values modified so that all the letters sort earlier than all the non-letters and so that a tilde sorts before anything, even the end of a part. For example, the following parts are in sorted order from earliest to latest: ~~, ~~a, ~, the empty part, a.