Once you hit 300+ sites, with attachments and custom fields, it starts to be one of those ‘I am going to pretend this will app work out if I ignore it’ things rather than an easy afternoon project.
I have more than 400 websites in mine, but not a single one has custom fields or attachments and I can't think of a single reason why that would be necessary.
If one does any of the following and wants to keep track of it in a structured way, it might require custom fields in some password managers:
* Use a different name for each account
* Use different "personal information" (date of birth, etc.) for every account
* Track "security" questions and randomly-generated answers for each account, for services that still use that terrible approach
* Track which phone number is associated with each account, for services that uses SMS MFA codes
* Attach list of one-time recovery codes to accounts that use those
* Attach source of credential information when credentials were sent by someone else for e.g. testing
There's six reasons off the top of my head. I'm sure there are more.
Some services use usernames for login instead of an email address, so I keep the per service email address in another field. Or I use a different name & birthday for a service etc.
I have a few that require custom fields. I don't really have a lot of passwords saved either. Maybe 50 tops.
Custom ones are usually all banking sites. One does not use standard field names so bitwarden does not detect it. Another has an extra field for user . (Bank customer company id, password then particular user's name).