Tony -lol.
I mostly start by a feel I get for a particular passage or idea and then if I carry it further I tap a tempo say into a guitar effector or my metronome which has a function in it that lets me tap a button and it shows me the tempo and this is how I decide on a project tempo. One of my last projects was at 111 BPM.
I have read that the reason for the common 120 tempo is based on a persons heart rate...well most people unless you are doing coke or something.
Give or take a few bpm, this is the tempo of your heart. Making songs in that tempo fit well into a persons inner rhythm. A little slower than that and the song takes on a more relaxed feel, tempos of 130 or higher are geared towards getting the heartrate up, the adrenalin flowing, like metal music.
A song written at say 126 bpm might be just enough to pull it out of limbo.Instead of the Sonar preset of 120.
So the tempo is a really big influence on how music feels.Something I intended to make more use of was to change tempos during a song because this adds more interest in some situations.
As for why certain bands might use an odd tempo of 119.83, I can't really say. The 48/44.1 khz conversion makes a lot of sense. Us musician types can have a ton of odd reasons why we do something.
I noticed quite awhile ago when I needed to play someone elses material that all kinds of goofy things were going on. A lot of bands seem to like playing in places that can be difficult to tune to or play.Maybe this is intentional to make it diificult to play their material or maybe they all tuned off of one guitar and it wasn't tuned up right or the guitar player likes alternate tunings....I know I ran off topic here sorry...on the subject of why some bands choose certain tempos it could be anything. If the band is into numerology they might decide to use only hex tunings and tempos( this isn't me BTW..not by a long shot) If 7 is significant someone might base an idea off of it. I once heard an album that was created based on the light wave lengths of the colors the rainbow....It is probably most often based on the feel of the music though.
Reasons are both scientific and unscientific for tempo choice. I am of the opinion that numbers are significant even if they seem random.