Hello!
I downloaded the MC 15w34a snapshot, then I downloaded the 15w34b snapshot when it came out.
I wanted to delete the folder of 15w34a in '.minecraft\versions', then I saw that the launcher deleted the 15w34a folder and replaced it with 15w34b.
I downloaded an older 1.9 snapshot, took a look at the versions folder again and I saw that the launcher replaced 15w34b's folder with the older snapshot's folder.
If this is meant to be a feature, or someone already reported this bug (and I didn't found it with search), I'm sorry...
This is intended to keep your hard disk clean. Snapshots are meant only to exist temporarily until the next one comes out.
If you want to keep snapshots, make dedicated profiles and do not use "Latest snapshot" as version.