B-factors in PDB files commonly are seen as a measure of (local) mobility in the (macro)molecule.When 3D structures are solved by crystallography, then, as the name suggests, crystals are needed. And in crystals by necessity, crystal artefacts are observed. The three most important artefacts are 1) crystal packing artefacts at those locations where the (macro)molecules interact with other (macro)molecules in the crystal; 2) parts of the crystal have a different conformation of the molecule (static disorder); 3) alternate locations when parts of the macromolecule are happy in multiple conformations (dynamic disorder). Crystal packing artefacts are a whole story in themselves and are not of relevance for the PDB. You find information about these artefacts in other PDB-facilities. Alternate locations are most often seen for amino acid side chains when these can have multiple rotamers that are energetically similarly favourable. If this is observed in the crystal, then you will find multiple side chain atoms with alternate location indicators A, B, etc. The occupancies normally add up to unity (Figure 3). If no alternates have been observed, the occupancy simply equals 1.00.

So, In summary, B factor tells about the possibility of different conformations and mobility of your protein. Yes, if you want to explain the three points mentioned above, then its really essential and compare to RMSF, provides additional insight.