Snails move at an incredible slow speed. An average adult moves at a speed of one millimetre per second.
While moving, snails leave behind a trail of slime. This is a lubricant that they produce to allow them to easily move across any type of terrain without injuring its body.
If they didn't lubricate the surface they move along, they’d simply scrape and hurt themselves along the way.
So, if you faced injuring yourself getting from A to B, I think you'd want to create a slimy trail of protection too!