Validation rules versus page-layout and field required
Key Takeaway
Page layout required works only at the presentation level of that layout. Only custom fields can be universally required. A validation rule applies to new and updated records, even when the fields it checks aren't on the page layout or in the API call.
Salesforce gives admins three ways to make users enter a value or enter good data: mark a field required on a page layout, make a custom field universally required, or write a validation rule. They differ in reach. Page layout required works only at the presentation level, universally required applies to nearly every way a record gets saved, and a validation rule checks any condition you can write as a formula before the save goes through.
Page layout required
A page layout controls which fields, buttons, and related lists appear on a record page. It also helps decide which fields are visible, read only, and required. Users get a page layout based on their profile, or on their profile plus the record type when the object uses record types.
Making a field required on a page layout makes users enter a value, but only at the presentation level of that layout.