I'm not sure why the subs thing works deeb but it definately makes a difference. I started a second team about 2 seasons ago and didn't use any for a while to see how I'd do, then started using them to see if there was a difference and tbh it was shocking how much better I was doing using them.
I can't explain it in any depth but one thing I know is that when using them there's no risk of an out of position sub replacing an injured player, and when you get an injury it doesn't affect the result. A serious injury sometimes does but not always from my experience. (But I'm no expert on how they work exactly)
To make sure you don't run the risk of a sub going into a wrong position you need a gk and an anyside defender, midfielder and striker if you play more than 1 upfront. You can only choose 5 subs and use 3 so it's impossible to cover every position with left and right defenders, midfielders, strikers and a goalkeeper. Obviously the anyside players can be hard to come by so using three subs when you can't cover your team properly with suitable subs would make complete sense. If you can cover them properly then you should do just as well without using the subs (in theory) but I'm still trying to work it out at the moment.
I will post some examples of how the three subs seems to help but it's a hard thing to explain exactly why it works, hopefully I can one day lol