The short answer
Yes — if you keep snails, a lid is strongly recommended. Several popular species are determined climbers that will scale the glass and go straight over the rim, ending up on the floor to dry out. Nerite snails are the worst offenders, and mystery snails also explore the waterline. A lid (or at least a well-fitted cover) keeps them safely inside, and mystery snails need that air gap under the lid to breed anyway.
Why snails climb out
Snails naturally graze up the glass toward the waterline, and some simply don’t stop at the top. Nerite snails are famous escape artists — they’ll cross the rim and wander off to dry out on the floor. Often it’s just restless exploring, but a snail repeatedly trying to leave can also be telling you the water quality is off, so test if the behaviour is constant.
Lids and the mystery snail exception
Mystery snails also travel to the waterline, and females deliberately climb above the water to lay their pink egg clutches. That means they need a lid and an air gap beneath it — too little space and there’s nowhere to lay; no lid and they may wander out. See do mystery snails need a male and female.
Keeping snails in safely
Use a lid with any gaps around filters and cables sealed off, and leave a 2–3 cm air gap between the water and the cover so climbing species (and any labyrinth fish) have breathing room. If persistent climbing continues, check parameters with a test kit — a happy snail in clean water is far less inclined to make a break for it.