Transporters are way less useful on their own than you think. Take the following scenario...

Centuries ago, your people developed transporters. You improved the tech until you could beam to the next star system. Now you have a network of them spanning hundreds of light years. You can cross your entire interstellar civilization in minutes. Your people discovered warp a couple decades ago, but it's merely a curiosity next to your transporters and wasn't developed much.

One day, you encounter a new alien race called the Romulans. They use primitive warp drive ships rather than transporters, so you don't think much of them. Things are a bit tense for a few years, and then they demand your unconditional submission to the Romulan Star Empire. This is absurd, so you obviously refuse.

Three days later, refugees start beaming in from one of the outer colonies. Reports indicate that none of your soldiers ever saw a Romulan. Rather than beaming down soldiers to fight, the Romulans levelled the colony with energy weapons from high orbit. Your forces tried to board the enemy ships, but they had some kind of energy field around them preventing transport. A lucky shot from a planet-side cannon firing beyond its rated range managed to find the mark, but was blocked by that same energy field just meters away from the hull.

It's been three weeks and now the Romulan fleet is in orbit of the homeworld. Bolts of green light start falling from the sky, obliterating the capital city, but leaving the capital building intact. Your transporters are still unable to pierce their shields. Your scientists think they'll crack it eventually, but they need weeks and you only have minutes.

With all the major population centers destroyed, the Romulan commander repeats their ultimatum: unconditional surrender or complete destruction. You accept their demands. Three Romulans beam into your office, the first time since the war began that your people have come face to face. Two are holding rifles. The other is holding a document and a pen.