The only way this belief is useful to me, is that it provides explanations to a few questions I would otherwise spend time answering; plus a wrong answer on them might make my real life worse.

First, avoiding generalized Pascal mugging: Yeah, everything is possible, including the chance that if I don't give you $1000 now, I will be tortured forever by an omnipotent sadist; but the probability is epsilon, so I won't give you those $1000 anyway.

Second, avoiding generalized quantum suicide: Yeah, whatever I do, in some universe it will have good consequences. And in some universe it will have bad consequences. But I should focus on whether the average (expected) results are positive or negative. For example, in case of a quantum suicide, the average result is me dead; in case of a lottery, the average result is not winning; in case of religion, the average result is no afterlife. On the other hand, when rationally doing useful things, the average result is more utilons.

The line between MWI and Tegmark Multiverse is not very clear, some of my arguments could be used for both. Using only MWI can answer questions about quantum randomness or generally about lawful randomness (which is probably on some level fueled by a quantum randomness: for example if I throw a coin, the exact movement of my muscles is determined by exact firing of my neurons, and a quantum event can make this signal a little bit weaker or stronger). But mere MWI cannot answer to questions like "what if this universe is just a simulation?", because that is outside of its framework (a simulation in what? possibly in a universe with different laws of physics? how do I calculate a probability of that?).