Word of mouth validates that you're keeping a promise worth making (and worth paying for). But, with a few exceptions, it's also a very slow way to grow. If you want to grow faster, then you need to market better and word of mouth isn't marketing. It's the natural (and unpredictable) consequence of delighting your customers.
Many SMEs don't market well, assuming they market at all. Most only do so when it's convenient (i.e. never) or absolutely essential (i.e. too late). Consequently, they get caught in feast or famine cycles: either desperately generating more demand, or struggling to keep up with the demand that they've generated. Few achieve an equilibrium between these extremes.
Most companies have administrative, operational, financial and employment systems. But very few have marketing systems, so very few market systematically. And without systematic marketing, it's very difficult to build a continuous pipeline of sales and escape the feast or famine cycle.
Marketing is the only business activity that directly drives growth. Everything else - operations, finance, admin, employment - is essential for sustaining growth, but is not a direct driver. If you aren't marketing every day, then you're squandering growth opportunities every day.
Most SMEs can't afford lavish promotional campaigns or world class creative talent. But however modest your marketing may be, doing it systematically will be much more effective than doing it sporadically.