Type of Mobility

Many people dislike slugs because of their appearance and plant-eating behavior. I can't say anything about slug's slimy bodies and protruding eyes.
When I was a child, I thought slugs were the children of snails, and I believed that one day a fine shell would grow out of a slug. Actually, they are different species, but both of them are members of the shellfish family that have made their way onto land and snails lost their shells during the evolutionary process and became slugs.
Snails have shells to protect their bodies against predator and drying. On the other hand, Slugs chose to remove their shells to move more easily even in narrow spaces and expand their living area. It's a very difficult decision to decide whether to give priority to protecting oneself of to expanding one's range of living area, but both snails and slugs are able to survive, so I think both decisions were correct.

By the way, there is a tradition of "Sansukumi" in Japan. It refers to a situation in which all three parties have one partner who is good at one thing and one partner who is bad at another, so that all three are stuck together. Slugs, toads and snakes are the representative example of Sansukumi. The slugs would be eaten by the frogs, but it was believed by the ancient Japanese people that their body mucus would dissolve the snakes.