The main reason for tuning your 808 kick drum is to make sure that your 808 sample plays in key with the rest of the instruments in the song.
For example, if the song is the key of D major you obviously want your 808 sample tuned in the same key of D major otherwise the discord will be apparent.
In other words you can perhaps think of an 808 sample as a “non-tuned” bass guitar whereby even if you play the right notes of the scale there will still be a lack of harmony between notes sounding together.
But this lack of harmony with the rest of the instruments can always be corrected by making sure you tune your 808 drums properly.
Eventually when you hit the note C you should also get an acknowledgement from a tuning audio plugin showing the same note you have just played.
