I have noticed in the slip area, where first slip stands way back from keeper and rest of the other slips like here:
Image source: Wikipedia.org
Why it is so? Eventually some times ball bounces in front of first slip,why they wont step up further?
Generally the outside edges from the bat, travel very fast. And these edges will be angled away from the keeper. So in order to adjust to the speed of the edge & to catch it safely, first slip will stand 1-2 steps back. And the tiny (thin) edges from the batsman's bat will not be diverted much. Which means, keeper can catch it. With gloves, it wouldn't be a big problem for the keeper to catch those thin edges. So keeper can stand up. Again, 2nd/3rd slip will be standing on par with the keeper. This is because, for the edges which are more diverted, angle the diversion saves the distance from the edge till the 2nd/3rd slip.
Two reasons that I know of off the top of my head. Firstly, to give the keeper space. The keeper can often take 1st slip catches if they are lucky. If the slip was in line with the keeper, they would collide if the ball went between them.
Another reason is that the 1st slip can act as a backup to the keeper if the ball goes between them. The keeper has gloves, so it is generally better for him to catch it than the un-gloved first slip.
The main reason for placing fielders in slips is that they can catch an edged ball which is beyond the wicket-keeper's reach. The gap between two fielders in slip position is minor enough that one slip fielder can catch other's fielder's catch chance. But they don't do that. But for wicket-keepers have to (and used to) dive to catch the ball which is in their reach. So if the first slip fielder is standing in the same line as wicket-keeper they may bump each other. That's why they place either behind the other fielders or (sometimes) ahead of them which is up to the bowler.