Sales are what keeps you in business so they should be your Nr 1 goal. That said, even if you are the best salesperson in the world, you need to keep your pipeline flowing in order to get those new sales opportunities. You need marketing to achieve that: expanding your audiences to reach new potential collectors, galleries and curators but also to make sure that your existing collectors know they spent their money wisely when they bought your art.
An artwork is not just a luxury good, it is also an aspirational purchase. When a collector buys a painting, he is not just attracted to the work’s wall-power and potential investment value but he also buys into the collector lifestyle (status, invites to art events, admiration from friends, etc). Therefore, your art requires context and third party validation.
Being represented by a (reputable) gallery achieves this to a large extent but you can also do a lot yourself through marketing: consistently communicating your achievements (sales, new works, exhibitions, reviews, etc) and your unique selling point as an artist. The reason that marketing needs to be consistent and long-term is that you want to keep yourself at the top of mind of your new and existing collectors, galleries and curators. Sales by themselves don’t achieve that as they are one-off events.
That said, I see artists getting frustrated when their marketing efforts do not immediately generate new sales and they give up. Or they want to do marketing two months before an exhibition opens and stop immediately afterwards. Neither approach is helpful. Marketing supports sales but it is not transactional like sales: successful marketing is more like an ongoing conversation over the course of years. Trial and error in this process is ok. Fortunately, marketing is no longer expensive: it is very affordable to create an online presence and communicate through social media and email newsletters.