You are a founder.
They are working with a technology or in an industry that you specifically want to work with and it is very hard to work on it professionally, and doing side projects are infeasible.
You need experience and you have no other option to get experience.
You are getting a significant title bump that moves your career forwards.
Invalid reasons for working at a startup:
Equity (getting rich off stock options) - this is very likely to be worthless unless you are a co-founder.
Salary - you would make (much) more at a big company.
Work life balance - you will work harder than you ever have.
Stability - does not exist.
Benefits - very bad.
Learning - they will not have any formal training nor time to train you, so be prepared to self-learn.
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All that said, I enjoyed my time as an employee at multiple startups. Just go in for the right reasons and your eyes wide open.