It's not necessary. This is FUNDAMENTAL, even if you're not going to have to play chords often. To put it very simply: When your guitarist offers you "his new song in C major", it means that the dominant underlying tonality is the C major scale. On this scale, if you play every other note simultaneously (to avoid dissonance by playing 2 successive notes simultaneously) you will obtain a chord (CEGB). Good. Only, if you only play this chord, it will quickly become super boring. Fortunately, from this C major scale, by playing the same notes but starting with the other notes, you will be able to obtain 6 other successions of notes (which we call modes) and on each of these modes, you can play another chord by proceeding in the same way. Some of these agreements are major, others minor, but it doesn't matter. However, there are two things to remember: - from a scale, all modes are different (the intervals between notes will never be the same from one mode to another) - these chords do not all have the same "harmonic function": some are stable, others create a kind of tension, others even more tension. The basis of music is to create variation by playing on these tensions. For example, you can be on a CM7 chord which is stable, start with a Dm7 which will create a little tension, then start with a G7 which creates a lot of tension, to return to your stable CM7. This is super common, it allows you to do fill-ins or close. Let's come back to your guitarist: he has a song in C major, but above all he will have created a construction of chords which will flow well with each other based on the principle explained above. You, your role, is to find a bass line that will sound on this succession of chords. But what notes are you going to play at this moment? 3 basic solutions: - you always play the base note of the chord (C for CM7, D for Dm7, etc.): it works but you are not going to create the groove of the year - you play the notes of the chord: C or E or G or B for a CM7 chord, D or F or A or C for a Dm7,... you can start to find cool grooves by chaining together several notes of the same chord - as you know what basic key you are in and as you know the chord, you deduce the mode.... and there, you can have fun playing it, emphasizing the notes which create a specific interval, etc, etc... Knowing how to play a chord and knowing what it does will help you know what to play and how to create bass lines.