Co-founders 'armothe' and 'redbean' spell it out on their website:
“We consider ourselves intermediaries between the community and the lumbering Danish company. For a long time the building experience has been the focus of creativity; but an increasing number of people are realizing their inner child through the role-play of the minifigs included in those sets. The mission of BrickForge is to fill in the gaps left open by LEGO and other brick-building companies.” – Armothe
“A lot of thought (and work!) goes into the creation of these tiny accessories. We do our best to consider ‘how would these look if LEGO made them’ and then we attempt to raise the bar even further.” – Redbean
And raise the bar they have! Looking at recent and future LEGO sets you can see areas where BrickForge tread first. LEGO minifig-scale pigs? Yes. WWII style army helmets? Yup. Centurion-style armor and weapons? Check!
This is where I see LEGO custom minifig developers working to keep 'the big guys' on their toes.