Targeting everyone seems like a good idea at first; it's a larger market.  But if we target just the people who we think will buy our product or service then there's a higher likelihood of conversion.

This can then also be fed back into any paid search campaigns or display adverts pushing people to the site.

If we're honest and tell people that the product or service is for a certain type or group of people then we're not wasting people's time and attracting that segment at the same time.

"Our products are for people who go to, or work at schools and colleges."
rather than
"We're for everyone in every industry."