I use it to get the B-V of the target star. I couldn’t find that in the AAVSO data.
Ah.
When I add a new star to my program, I go to https://app.aavso.org/vsp/ and fill in the appropriate fields (Star, FOV, orientation, limiting magnitude, and more) and first get a finder chart, which I print for use at the telescope.
Next the photometry for the sequence in the filters required is downloaded. The B-V values and their errors are provided whether you want them or not, in line with the bottom of the VSP page where we find the words: “V and (B-V) magnitudes are always displayed. Select any other bands you wish displayed below.” A Perl script subsequent mungs the photometry data into a format useful for my current processing pipeline.
I never take a given B-V of the target star as gospel because it can itself vary as the brightness changes. If I want to know a colour index of a VS at a particular epoch I measure it for myself.