I acquired my first 7-string guitar in 1990, a ’71 Gretsch “Van Eps”.
George Van Eps is by most regarded as the “father” of the 7-string jazz guitar with an added low A-string. He called it his “Lap Piano”. George, although deceased in '98, can still be seen playing his Gretsch 7-string on YouTube, where you, by the way, can find many more jazz players performing on the 7-string guitar.
The 7-string guitar creates the possibility to play chord-melody with more true sounding bass notes. As the extra low A-string is tuned the same as the 5th string on the regular guitar, only an octave lower, many chords forms can be played on the 7-string by just moving the finger from the 5th string to the 7th string. Of course this is not possible for all chord forms, so there is some learning of new chords (grips) involved. Also the visual part of seeing the finger board with the added lower strings takes some time to getting used to.
If you play three voice Freddie Green style rhythm chords (6th, 4th and 3rd string), you have the added possibility to combine these with three voice chords on the 7th, 5th and 4rd string. This again will require learning some new chord shapes (grips).
I did not really get "going" on the 7-string until I got an Ibanez AF 707 jazz box in 1998 and gradually I switched completely to the 7-string.
Many luthiers offer the 7-string, either as option, or as a regular offer; e.g. Bob Benedetto, Frans Elferink, Jimmy Foster, C B Hill, Stephen Holst, Chris Larkin, Bill Moll, Dale Unger, Bill Moll, Nelson Palen, Stefan Sonntag and many more. Also factory built guitars like Robert Conti, Ibanez, Eastman offer great 7-string guitars.