[AYBABTME]

Another reason could be: if you systematically scrub everything, then a third party wouldn't be able to use the fact that you scrubbed something as a hint that the thing had valuable information in it.

e.g.: if you use an encrypted chat app ONLY after a murder occurred, then this would be suspicious. If you always use encrypted chat apps, then there's no information hint being given about your possible involvement with the murder.