The main function of a fibrous joint is to

In a fibrous joint the bones which are adjacent to each other are connected by fibrous connective tissues, mainly collagen, with out forming any cavity of joint.

Explanation:

The fibrous joints and not movable and cover important structures like brain being covered by skull bones which are articulated by fibrous joints namely sutures.Long bones too are stabilized by this type fibrous joints as is formed between radius and ulnar bone of forearm.Another important fibrous joint is in between the root of the teeth and gum sockets in maxillary region. Here fibrous joint binds the teeth firmly so that continuous motion and pressure during eating may not loosen it.

The answer is centrioles forming happens in animal cell mitosis but doesn't happen in plant cell mitosis.