If your business owns a dollar it didn't have before, it's worth a dollar more.
But you aren't worth any more, because that dollar bought equity: in theory, the value of your holding is just what it was before.
So there are three key factors on the side of taking in money.
First, in the world of startups, value is much less cut-and-dried than in established businesses, where it's essentially a function of business shape and performance. Assuming that you only take in money that recognises increasing value, this 'evidences' value growth. And yes, this is also how bubbles happen. But your start-up will create strong value, right?
Second, more cash means more choices. You can use that cash to run on a little longer - so it's less likely you'll run out of cash en route to monetization. Or you can change some compromises you were making that preserved cash but cost time.
Third, more investors give you more connections. And, if you've qualified your investors carefully, this will give you better access to your market - or whatever else it is you know you do/will need.
There can be downsides too, of course. But that wasn't your question.