You renege when you accept an offer and later tell the employer that you will not join. This is different from declining an offer before its deadline or withdrawing from interviews before accepting. It can happen with an internship or a full-time role.
Banks care because recruiting classes are planned well in advance. Once you accept, the team may stop interviewing candidates, reject alternates, assign headcount, and begin onboarding. A late withdrawal leaves the bank with less time and fewer candidates to fill the seat.
Reneging is therefore a reputational decision, not merely an administrative one. Even if your employment would be at will, meaning either side can generally end the employment relationship, that does not guarantee freedom from every consequence. Your offer documents may contain provisions concerning signing bonuses, relocation payments, confidentiality, immigration support, or repayment obligations. Review what you signed rather than assuming there is no contractual issue.