It does a lot of good. Every little intervention really helps, even if it's seems far from perfect.
It's because the system as a whole is balanced on a critical threshold. It's like water boils at 101°C and condenses at 99°C.
The point is to reduce R below 1. When R > 1 the disease gets almost everywhere eventually. When R Currently R is above 1, but it's not a lot above 1. (We're lucky. There are other diseases where it's much higher.)
So even if you have been spreading on those 4 days, you are not spreading from days 5 onwards. That might be 10 days less spreading.
You are not giving it to large numbers of people in those 4 days either. As a spreader, you're adding risk to others, but only 1 or 2 will get it directly from you probably.
When you isolate on day 5, that's a reduction in the amount of spreading and it reduces R. When your traces also isolate from your day 5, that's a further reduction in the amount of spreading and R. That's recursive, if their traces also isolate, it's a further reduction, etc.
Altogether, if everyone follows these changes, there's a reasonable prospect of R falling below 1 enough for the disease to fade out. Because it's close enough to 1 already, that behaviour can make a significant difference.
In practice people do not comply, and they lie sometimes (https://www.bbc.co.uk/news/world-australia-55011790). So R does not fall as effectively as it would if there was high compliance. But every bit of compliance helps reduce R.
When R is consistently It looks unlikely this will happen until we have widespread vaccination in some countries (USA is one of the leading examples), but the more we reduce R meanwhile, the fewer people will die or get long covid in the end. And fewer here means in the millions.