Entering building names can be a bit of a pain. I'm not even sure if it's required - according to Wikipedia it looks like you can just keep adding dashes until you eventually get to room number (e.g. 4-5-10-103) [1]. But a lot of address forms ask for it, so I end up entering it anyway.
[1] https://en.wikipedia.org/wiki/Japanese_addressing_system
The building name is required in many cases because two buildings next to each other will often have the same ban-chi-go (i.e. 4-5-10 could refer to two apartment buildings that were originally on the same lot, both with room 103).
It's exceedingly common, not just some edge-case you see every once in a while.