It can be a little of both and vegetarians/vegans don't all have the same mindset. There are different reasons people decide to not eat meat or related things. Just because they don't want meat doesn't necessarily mean they only want vegetables.

For many there is a desire for a protein source that has the texture of protein-rich foods. There is also familiarity as most people grow up eating the the things mock meat imitates.

Well for one, not everyone is able to eat eggs or dairy. For these people (such as myself), there's not an alternative besides a plant based substitute. To me it's pretty exciting to see new developments here. People in this category probably aren't a large portion of their market though.

I'm sure there are some who do it that way, but it's not the point for most. Animal-based foods are dense with nutrition and tasty. If you could have that without harming your health, or that of the environment or animals, why wouldn't you?

Some people prefer not to be reminded of animals at all, and they'll adopt a diet that doesn't resemble them. They don't need a "stepping stone" to it, though sometimes they could use some assistance learning how to eat a nutritionally complete diet. It's perfectly possible; it just requires a bit more thought.

It's not clear that these products actually are better for your health or the environment, but it's quite clear that they do reduce animal suffering. For many, that's sufficient.

It's a bridge/comfort thing until you've learned to cook with a new set of ingredients.

Once you can properly season your tofu, sietan etc... the imitation meats kind of just become a novelty.