Before now, word of mouth do the work of marketing, by this I mean the referral method and it's a really good one that still works today for marketing, technology has advanced in a really good way making everyone a content creator for their brand and for those that can afford influencer can do it. Marketing is good but the cost of doing one should not be more than profit to attain.
Word of mouth marketing is very effective and now thanks to social media is even more effective than before, after all if you have a friend which tried a product or service and you can see it was a good value for their money then if you have a need for that service then most likely you will ask your friend for a way to contact and hire the same company, however while word of mouth is effective, it is also not very scalable, and as such other marketing methods are used by companies as a way to gain more clients.