Why are follow-me cars used in Gran Canaria Airport in good visibility for incoming aircraft?
The unspoken truth (no references, sorry) for this also applies to many airports around the world (not just Gran Canaria). The follow-me service is not free, and it's one of the ways airport operators make money and create jobs for the community.



The Spanish Aeronautical Information Publication (.pdf) for GCLP does not state a reason, merely confirms what you saw, a follow me for all arrivals regardless of LVP. Here's an example of such paragraph:


  ATC clearance to land at RWY 03L, leave runway via TWY R1R or R1L, following TWY R2 and wait for ATC clearance to enter apron via Gate J, following the instructions of "FOLLOW ME" vehicle, via TWY R10 to the stand (...)




Also see a related discussion on pprune.org.