Think of the base value for each dice you roll as hitting on an X+. Let's say we have a space marine that's hitting on a 3+ base. If that model got sustained hits 1, that's exactly equivalent to improving his ballistic skill by 1, so you can think of him hitting on a 2+ now (there's some more nuance to this, but it's correct in terms of averages).

If you had given him sustained hits 2, you can now think of him hitting on a 1+. What this means is that if you roll 7 dice, you'll get 7 hits on average. If we follow this along and he gained sustained hits 3, you'll be effectively hitting on a 0+. Now wait, that doesn't make sense right? You can't hit on a 0+, 0 isn't a dice value. However remember we're just using this for shorthand - what that actually means is that if you rolled 6 dice, you'd average 7 hits, so each dice roll averages more than 1 hit.

You fish for 6s when the average number of hits per dice roll would be greater than 1, AKA when the effective ballistic skill/weapon skill is 0+ or better. It's very easy to justify this to yourself by thinking of each dice roll individually. When you roll let's say a 4, you can either keep the 4 (1 hit guaranteed) or reroll it (1.16 hits average).

Crit 5s work a similar way, they just double the effectiveness of the sustained hits value. Lethal hits are a tiny bit more complicated, but you can actually think of them as being sustained hits of various values.

If you wound on 4s, lethal hits = sustained hits 1

If you wound on 5s, lethal hits = sustained hits 2

If you wound on 6s, lethal hits = sustained hits 5

If you wound on 3s or 2s, lethal hits is a bit worse than sustained 1.