Over 30 views no replies, so I'll have a go.
Basically comes down to looking at risk vs reward and realistically thinking about your chance of success.
Risk vs Reward ?
Well you're taking a risk by putting a player in an unfavourable situation, by trying to make him the enemy's focus. What do you want in return (reward?) Depending on the situation, maybe one or more of: a key pick, some good initation damage, allowing the rest of your team to get position, forcing the other teams uber etc ?
e.g Situation:
Granary last. Both teams have even uber and 6 players up. You want to push last.
Often teams will sacrifce soldier(s) and scout(s) to force the enemy medic to pop uber, whilst keeping their own medic/demo (and maybe a soldier) safe inside cp2.
You're taking a risk, in that you're sending in 3 or 4 of your team in to a fight vs 6 and need them to force the uber, otherwise you'll be at a large player disadvantage. The risk is offset by the potential reward that if your suicide run is successful, your demoman+medic (and maybe a soldier) can defend/delay granary 2nd long enough, for your respawners to come back up, and can push with uber advantage.
Chance of success?
Helps if you're unpredicatble. Mid for example. If your player bombs on to the other medics head on every single granary rollout, the other team is going to eventually counter it and have a scout waiting to smash him (or they'll change rollout and the medic will be looking for it as well etc). So the chance of you getting away with it should diminish (assuming equally skilled teams).
Specifically (paraphrasing a div1 soldier

) "with granary mid the intent isn't to suicide your secondary". But if you have the opportunity to jump on the opponents half of mid (crates even catwalk) you basically draw major attention to yourself, and hopefully (whilst dealing some damage), may take some heat off your demoman. That's not saying don't "man up" though, if an opportunity presents itself
Hopefully the link works ?
https://www.youtube.com/watch?v=kLqWmzQT5l4
The opening play on that was from owl6 go vs foxy. SoD opens up with a solo distraction jump down the garage side. The intent for my team was to look at how we could deal with it and what we did wrong (too many people moving to cover in response and we end up clumping in death's corner).
But the intent of that first clip for you is to note that SoD himself doesn't get a kill, but instead allows his demoman to take prime position on the point which goes a long way to winning his team that rollout.
The rest of the clip is just examples of some other jumps which I think of as 'other than default' which you see on granary mainly from a sollys named relic/platinum. Where teams are rolling their soldier/s in to positions other than just the usual close crate or T-crate, or bombing on the medics head. i.e. The one's where soldier's actually rollout from catwalk and go to the other teams crates or catwalk, cause major distractions, they don't always work, but atleast it's part of a plan and "something different".