What's the reason for the TCustomXXX classes in Delphi class hierarchy?

This is probably somewhat dumb or even explained in the docs but I couldn't find it.

There's a pattern that seems to repeat among a lot of Delphi classes which is the creation of a TCustomSomething before the actual TSomething. I wonder if there's any special reason for that and in case I want to create a derivation of TSomething should I inherit from TCustomSomething or directly from TSomething?

I can give some examples:

1. The TStringStream inherits from TByteStream that inherits from TMemoryStream that finally inherits TCustomMemoryStream.
2. TRESTClient directly inherits from TCustomRESTClient.
3. TButton inherits from TCustomButtom.

There are indeed cases I see the actual benefit from the base TCustom... class, like in TMemo and TRichEdit, but I fail to understand why TRichEdit inherits from TCustomRichEdit instead of directly from TCustomMemo.

TL;DR: What's the use of the TCustomFoo classes in Delphi? When extending an existing class should I inherit from TCustomFoo or from TFoo directly?

You can inherit from either, but you'll often find that the TFoo class adds no functionality to the TCustomFoo class except for making protected properties public or published, and/or making public properties into published properties.

You may want your own TAraujoFoo components to not have all those properties public/published, in which case you can inherit from TCustomFoo instead of TFoo, and just make public/published the properties that you want TAraujoFoo to have.