Fullers are used for isolating steel, and spreading metal Along its length or to widen it.  Really... anytime you require precise hammer blows.
The handled fuller is used by itself or in conjunction  with the bottom hardy.

This set  of the two fullers is 1/2” wide and well radiused and works well for hammer and axe making. A top and bottom make up this set . Made to fit 1” square hole. 
pick them up in person at https://www.frontstepforge.com 

or online here. Want a size not listed ?send me a message at spc@frontstepforge.com