Tread boards in external stairs for houses
Spacing between the tread pieces allows for water drainage and for the wood in that area to dry out. And yes the spacings between the tread pieces also help with the slip resistance. 

It is common to use multiple narrower pieces for outdoor decking stair treads because they will tend to stay flatter overall when the wood has been exposed to the elements over time. If you use tread pieces that are full width lumber there is much greater chance that it will twist and warp over time. A wider flat piece can also get quite slippery when it is wet.

Another thing to consider, if you are using treated lumber, is that the chemicals in the wood will penetrate to a greater percentage of the cross section of narrower pieces. This will help to preserve the wood over a longer period of time.