Collenchyma and sclerenchyma fibers both have a similar main function. what is it?

To provide mechanical support to the plant parts

Explanation:

Collenchyma fibers consist of living cells with unevenly thickened primary cell walls and mainly serve to provide support to the plant parts of herbs. These fibers provide elastic support and are also present in the form of long strands near the surface of stems and along the leaf veins.  

Sclerenchyma fibers consist of dead cells at maturity and have an extremely thick cell wall. The main function of these fibers is to provide strength and support to the plant body. The presence of thick secondary cell walls in cells of sclerenchymatous fibers makes them strong and hard to provide mechanical support and strength to plant part. These fibers are particularly abundant in stems of woody trees and certain types of leaves.