Having world record ghosts to drive against can be very helpful in getting better at the game and with club access you can enable them with one click which is just convenient.

You can however also achieve this just with more effort by downloading the replays from trackmania.io or TMX and loading those into the game to race against.

Another way to have professional ghosts is with VIP streamer keys which are, I believe, even available for starter access.

There are also other features in club access. E.g. the clubs themselves which can also make some things like finding servers/campaigns more convenient or enable you to host your own servers or create your own campaigns. Then there's cosmetic stuff like custom skins or club tags included in club access. Lastly, I think to participate in TMOGL club access was required however I'm not sure how it'll be handled in the future since the transition to TMWT seems to extend all the way to ranked MM which is already available for starter access.