Well, Leggy appears if Meggy gets too scared. But like Spiderman, Peter Parker took time getting used to the fact that webs shoot from his wrists and that he can climb and stick to surfaces. If Meggy gets scared enough, she might be able to become Leggy on command, allowing her to go places Meggy can't. Once Leggy gets to where she wanted to go, she can become Meggy, putting Meggy in a place only Leggy can access