Custom fields are how a tracker fits your process. They are also how many Jira sites end up with hundreds of fields that nobody remembers creating.
Per project by design
In Starlex, custom fields belong to a project. One team's “Customer” field never appears on another team's issues, and removing a field only affects the project that owns it.
Useful everywhere
Custom fields show in the issue panel and are imported from CSV by name. In issue search you can filter by them, for example customer:acme, add conditions on them and save the filter as a view. They also export with the CSV.
Votes and flags included
Common extras such as votes, flags and start dates are built in rather than added as fields, so they behave the same in every project.