Why is glycerol supplied/used as 87% solution?
Typically, a glycerol stock will have a final concentration of 10-20% of glycerol. Glycerol is highly viscous. Pipetting higher concentrations of glycerol is difficult (especially for smaller volumes). Therefore a lower concentration such as a 50% solution (works for me) is convenient: add 300µl to 700µl of bacterial culture. 

Regarding 87%: some vendors provide 87% glycerol along with anhydrous (>99%) glycerol. The reason is probably the same: it is easier to pipette 87%. 

From a vendor's website:


  Glycerol is a component of several buffers, media or reagents in
  biological research. It is supplied as glycerol anhydrous or 87 %, the
  latter containing 13 % water. Both glycerols are from the same
  quality, the 87 % glycerol has the advantage of a lower viscosity.
  Anhydrous glycerol can be pipetted with difficulty only, especially at
  low temperatures.


But the question is why 87% and not 90 or 80? See Michal et al. (2017)  for a relationship between viscosity and concentration of glycerol-water mixtures. 



Another graph plotted from the data given in the document mentioned by iayork in the comments. Different coloured lines denote data at different temperatures (in °C). Original data from: The properties of ordinary water substance. Compiled by N. E. Dorsey, New York (Reinhold), 1940.



So I am guessing that 87% must have been optimal beyond which the viscosity just shoots up rapidly with a small increase in concentration. I wish to reiterate that this is just my guess. I could not find a authoritative (in fact any) reasoning on why precisely 87% glycerol is (or was historically) supplied.