Pushchair

Ive never got why the mose basket on pushchair does have straps ? I’m having my third and don’t remember using the Moses basket/ having no strap on them both there seat alway went flat I know they need to be flat. But wouldn’t it be safer if they were strap in ? I’m worry that if by any chance the pushchair gets know she fall out or would she be sliding down as I got up crubs/hills and stuff.