In U.S. politics (as well as in some other countries utilizing the presidential system), the majority floor leader is a partisan position in a legislative body.
Thus, the majority leader is seen as the de facto leader of the Senate, especially in modern times, and thus, in accordance with Senate rules, the presiding officer of the day gives the majority leader priority in obtaining recognition to speak on the floor of the Senate and they determine which bills get voted on.
[1] In the United States Senate, the current majority leader is John Thune (R-SD), who was sworn in on January 3rd, 2025.
In the House of Representatives the majority leader's presence and power often depends on the session.
In the United States House of Representatives, the current majority leader is Steve Scalise from Louisiana's 1st District.