Everytime someone told me that they want to sell something:

"So, who's your target market?"

The more specific it gets, the better the chance.

If you want to hunt a deer, bring bow or gun. Not stone.

That's the importance of choosing a target, to take one step ahead. Prepared.

Better example:

If you know you want to go to a mountain, you'll need a jacket to stay warm. Shirt might help you a bit, but it's not effective.

Just like when you know your target market, you'll use the right method to get to them. Effectively and efficiently.

Why it needs to be specific?

It will save you from unneeded pain.

You just need to target one demographic for an ads and content. And you just need to tailor your products or service to match one specific demographic.

You don't have to waste money and energy unnecessarily.

What if there's another demographic that you missed out?

You already build your demographic variable, you just need to adjust.

But you don't know it from the beginning, you only know it once you put your product out there.

Example:

You targeted a college girl for your product, so you tailor everything to match those user persona.

But, when you launch the product, it turns out that you have another persona that use your product. You can leave the first target and turn to second one. Cont...

Or, you can tailor for both if both of them not really different.

You got your target market, then adjust.

The less variable you adjust, the better your chance to reach your sweet spot.

How to have less variable? By having specific target market.

Less is better.

But, this thread would mean less if you can waste a lot of money and a lot of time. In other words, you have unlimited resources.

Some people have that privilege, some just don't. And I think it's better to move in an efficient and effective manners.