some children need them some don't - mine have different in "compliance" the least compliant ended up strapped in a pushchair for as long as possible.
My friends near 4 year old, always compliant, always walked nicely next to the pushchair for nearly 2 years, never ran off, suddenly ran across the road and got knocked over a few months ago. He is fine but it just shows you doesn't it - they can't even work out why he did it - didn't see anyone/anything, weren't waiting to cross the road etc.