If you have a dog with an injured paw then different people will willingly help you (dog lovers) than those people that will willingly help you find a lost cat (cat lovers). Same with directions, some random tourist might not be as willing to help you with directions as a native, the tourist could probably get the maps out on their phone, but they will not feel as if that is the thing to do. However, if you were lost in their home town then they might walk you to where you needed to go.
As a cyclist I often help other cyclists with flat tyres etc. and you try stopping me from helping out. I spent a decade working in the cycle trade so, for me, fixing someone else's bike is merely an opportunity to keep my skills fresh. I cannot say that first aid is my thing, however, I am sure that someone else will keep the patient warm until the authorities arrive, so I auto-delegate myself to warning traffic and collecting debris from the road in such circumstances. Because I don't do cars, I am unlikely to offer to bump start someone's car or help them back in if they have locked their keys inside. However, others that have those skills will take up the opportunity to help.
When it comes to violent confrontational circumstances, there was a time when I would have a go, however, after getting my head kicked in by four muggers I no longer feel inclined to get involved. However, I do have friends that have been doormen and they will leap to the opportunity to do a little bit of enforcing - it is in their skill set.
Most people are willing and able to help out if they can and if they have confidence in their usefulness in a given situation. The problem is that when there is some rare and outrageous act of violence going on that there are very few people around who have 'enforcing' in their skill set. Hence the apparent 'bystander effect'.