First things first: what is a meaningful fanbase? A meaningful fanbase is a fanbase that that reads, enjoys, and most importantly, interacts with your content. This means that the fan followed you on the basis of having common interests, and likely posts about similar topics - or is a friend of yours. The fan is also an active member of social media themselves, and has probably has their own following.
For good measure, let's talk about what a meaningful fan is not. He or she could care less about what you post, not because he doesn't think the content is quality, but simply because he is not interested in the subject. He or she owns the social media account, but isn't active. Worst of all, he or she - or it, in this case - could be a spammy, bot account.
So why are meaningful fanbases so important to cultivate?
1. You are your own brand. This means you need to have your own brand advocates. Who better than the people who follow you? They are the people who know the "product" - your ideas, your content, your words and personality - you put out the best. Without a meaningful fanbase, you lack brand advocates.
2. Your feed gets repetitive if no one interacts with you. Now this is ultimately comes down to personal preference, but there's definitely a wrong way to do it. For instance, a Twitter stream that consists of solely links and content is boring. Readers want to see the human behind the tweets too - it makes you more relatable. On the other hand, no one wants to read a brand's Twitter feed that's all replies to people.
3. You never get real feedback, so you never get to change for the better. It's one thing for someone who barely glances at your posts to give you "positive" feedback. It's another for someone who peruses them deeply. One is valuable and one is just empty flattery. One will help you become a better marketer, writer, or whatever it is you are - provided you know how to take criticism. One won't.