Dorothea Salo notes the apparent perversity of the first indicator of the MARC 245 field being set to 0 to indicate the title as main entry and 1 to indicate that it is not. At the risk of being picky, it is worth pointing out that the first indicator does in fact mean “should there be an added entry for the title?” rather than “is the title the main entry?” so does in fact keep to the logical 1 for true and 0 for false. See the wording of the MARC21 concise manual.
My understanding of the problem is as follows, which makes much more sense if you think in terms of printed card or dictionary catalogues, which MARC was intended to help produce, and in an age where it is wise to save computer memory. The main entry heading in MARC would always be given an entry. In a card catalogue, for instance, it would be nonsense not to. Sometimes the 245 would automatically be given an entry because it would be the main entry; sometimes however- when there was a 100 present- it wouldn’t be. In this case, the cataloguer needed to make the first indicator one in order to make sure it is given an added entry which, for a card catalogue, would refer back to the main entry record (under author, uniform title, etc.).
This system is now bonkers and largely irrelavent. There are no longer multiple entries for a record each referring to the main entry which holds the full catalogue record: the OPAC renders this pointless. The original purpose of MARC was in the preparation of printed catalogues where it was useful. Why this hasn’t been reformed by now I don’t know.