A build-up of saliva in the leadpipe can lead to a build-up of corrosive material. At best this can be unhygienic and give your case a musty smell. At worst it can lead to red rot which eats through your leadpipe causing tiny pin-prick holes that leak air.
This silk swab is affordable, quick to use, and it isn’t hard to add it to your routine. It can also dry out your main tuning slide effectively, the second dampest part of the instrument.