Erm, indoors...
Depending on building construction, signal attenuation can vary dramatically - some buildings don't have much impact, but my wee flat, constructed in the 19th century, with 40cm stone walls, which then have insulation panels that are backed with aluminised mylar film fitted internally - well, let's just say that I go from 5 bars outside the front door*, to zero bars sitting in my arm chair - a grand total of 3meters away.
If I leave the phone by the main window in the bedroom, I can get a sort of signal.
Wi-fi coverage is the same - I have to move the router to the window if I want to go sit in the garden with my laptop.
Also, outdoors, you can get enormous variation in the signal levels with shadowing and other such wondrous imponderables (know unknowns as certain vice-presidents would say).
This tool presents a general overview of the signal in an area. It will never have sufficiently fine granularity for the precision a lot of folk here think it should have.
I haven't seen a similar system on other providers, and, IMHO, it's not a bad first approximation of a useful tool, provided people look on it reallistically.
*I can see my nearest base station from just outside the front door