We’ll admit, there’s not always a concrete explanation for things in Russian Doll, but if we had to summarize the reason Nadia and Alan time travel, it’s to make peace with their family history, as well as with the events (and the outcomes) they have no control over.
Regarding Back to the Future and The Butterfly Effect, time-traveling protagonists Marty McFly (Michael J. Fox) and Evan Treborn (Ashton Kutcher) are able to drastically change the future with the most subtle alterations of past events — which becomes a complete mess in both films. It’s not quite like that in Russian Doll.
Source: https://www.distractify.com/p/russian-doll-time-travel-explained