I would like to add some more insights to the answer that was already given.
Gaining match practice as a reason to enter as a qualifier
First, I would like to mention that forgetting to sign in for a tournament on-time is only one reason for top seeds (in the main draw) who play qualifications. There is also another reason: Players sometimes do this on purpose if they want to gain match practice, that is, they want to play as many matches as possible.
For example, in 2016 Dennis Novak played 4 Futures (more or less) back to back and in each he played qualifications, despite the fact that he would have been top seed in the main draw. I think the excuse that he didn't sign up in time, is not valid in this case.
Here are the draws:
Tunisia F14 Futures Qualies (draw)
Tunisia F14 (draw) Futures Main Draw
Tunisia F15 (draw) Futures Qualies (he lost!)
Turkey F18 (draw) Futures Qualies
Turkey F18 (draw) Futures Main Draw
Turkey F19 (draw) Futures Qualies
Turkey F19 (draw) Futures Main Draw
Entrance as a WC into qualies
I would also like to stress, that only in Tunisia F14 Futures, Dennis received a Wild Card into the qualifications. For all three other tournaments he entered as a normal player (without a Wild Card). I am not sure why he needed (?) a Wild Card to play the qualies of Tunisia F14 Futures, because there is also the option to be on-site at the qualifications deadline. In that case (irrespective of your ranking) you can play qualies.
After reading the rulebook section regarding entrance into qualifying draws, I am pretty sure that entering via Wild Card is done, when there is the possibility that the Qualifying Draw will be full with regular players. In this case, a top seed player, who wants to play qualificiations, can only play qualifications with a Wild Card. If the player can be 100% sure that the qualifications draw will NOT be full, he does not need a Wild Card and can enter the qualifications draw as a so called On-Site Alternate.
Top-Seed plays qualies and main draw is already done
Another thing I would like to note is that usually, if a top-seed, who would be (top) seeded in the main draw, plays qualies, the Main Draw is not made until the qualies are finished. The reason for this is, that the top-seeded qualies player will then be also the top seed in the main draw. However sometimes, the Main Draw is already made before the qualifications are finished. In this case, the person, who would have been the top seed in the main draw, if the organizers would have waited until the qualifications are finished, will just be a regular qualifier. There is a valid chance, that this regular qualifier will play a seed in the first round of the tournament (which is stupid, if you ask me).
This case will likely happen this week in the Italy F9 Futures (draw), where Stefanos Tsitsipas (ATP #198) is currently playing qualies, but the Main Draw has already been made. Let's see who he will face in the Main Draw (if he successfully qualifies).
On the popular tennisforum Menstennisforums there are some threads related to this phenomenon: