I am currently using Tighnari for exploration in Sumeru because I got him early on the banner. However, if I had not gotten Tighnari, I planned to use Collei in the first place. Usually, it happens that free character at the start of a new region is really good for exploration. Like Beidou was given for free during Inazuma, there might be puzzles or exploration objectives that require that element's character. I agree that we have Traveler, who has access to every element released for them, but having an extra character from the same element is a perk. There are so many objectives in Sumeru that require a bow character, and Collei being a Dendro bow character, helps out with that. For instance, there are Withering zones that you have to clear after attacking the Withering plants with Dendrogranum. Using a bow character helps to clear that fast because you can maintain a distance from the enemies and avoid falling down quickly.