K8s works best (economically) when it can bin pack things. The only way it can bin pack things safely is by having user provided limits--its not smart enough to right size your app (out of the box at least). Not setting them means you're going to end up paying more or have resource contention/outages.

It's better to set the limits higher than you need than to not set them at all. Ideally this is easily done since you're profiling/load testing your app and you understand the appropriate sizing for it, right?