Tach adapters come in two basic types - I've designed one of each before. Filters take a signal off the coil, remove high voltage spikes and short pulses, and send out a cleaned up 0-12 volt signal. You use these if the tach isn't steady, reads too high, or for some non tach application like triggering an ECU off a line lock. If a ready to run distributor uses a tach filter, it's one of these - and it is likely to work without it.