The lead source records how an account came to you: a referral, a trade show, the website, a cold call. It is one field, and it is the only one that can eventually tell you which of those is worth doing again.
Setting it
Set it on the company's Key Information card, in bulk from the companies list, or from a column when you import. Set it as the account is created — a lead source added six months later is a guess.
One catalogue, every tribe
Lead sources are defined once for the organisation, so every tribe picks from the same list. That is what makes it possible to ask "how much did trade shows bring the network?" and get an answer instead of forty spellings.
Filling the gaps
Accounts with no source show as Unattributed in the reporting. A few are unavoidable; a lot means the reporting cannot be trusted. Filter the companies list for a blank lead source once a month and fix what you can still remember.