For a small company that is just starting out selling, it is impossible to reach out to the world. Not enough people, not enough money, not enough time.
Think about something that people usually use in manufacturing: economies of scale. One approach: invest a huge amount of fixed cost to build a massive plant that then can churn out products at a very low cost per unit (huge fixed cost / huge production volume). The other approach is specialisation. By focussing on just one tiny product in a tiny product segment, you might get to the same scale for that product as big competitors do who have to worry about 5,000 other items.
The same is true in sales pitches. You can scatter yourself and pitch clients in different industries, different continents, of different sizes. Or you can just focus on one industry, learn the “language” the people speak there, understand their problems, get referrals to from one happy client to another.
Which sector? Obviously it should make sense given your product. But after that, it is probably up to you. I guess that most companies that follow this approach got there by coincidence. Somehow you start winning pitches and getting leads in one sector and run out of time to focus on another.