Many groups have been demolished through poor leadership. Nighttime assaults risk great disaster if there is gross decentralization of leadership and everyone is following incompatible orders. No one likes being told what to do or how to play a game so leading an adventuring party is tricky business. Why parties benefit from leaders and two solutions to the hierarchy problem are explained below.
It's easiest to figure out when a group needs a leader by thinking about when it doesn't. If each of the players understands the others well enough for a group strategy to form on it's own, you're good. When battle plans are executed well because everyone already grasps the role they fill and knows how best to fill it then no independent leader is necessary. A lack of player-versus-player conflict is also a good sign the group functions well enough on it's own. In essence the party has a solid respect for the unspoken leadership developed between the players and this means no tactician is required.
Leaders aren't necessarily responsible for making every decision, however. Some choices are made easier through centralized guidance - when the party may face their demise, perhaps, or when pulling off a particularly tricky tactic. I have yet to meet any players who prefer their party leader to micromanage them. The party leader could just be the one who breaks tied votes in a democratic party, like a chairperson. They could also be the person that important NPCs contact on behalf of the party. Different groups prefer to delegate different roles.
Natural leaders are fun to come by. Sometimes a person is thrust into the role either implicitly or by announcement. The atmosphere of the campaign can do this as well. An experienced player may find themselves there as others look to them for the rules. When this occurs a DM only need accept and encourage it. So long as the natural leader is helping keep the game fun for everyone then they're a great asset to the group.
My preferred method is shared leadership. This happens when each person in the party has their own specialty: when the game is in their domain, everyone trusts their judgment. Either in-game or out-of-game specialties come in handy. The party rogue who has already scouted the enemy stronghold makes a good leader for that part of the adventure. Mapmakers are good candidates for choosing which quests to do next if those quests are prioritized by relative location to the party. More involvement by each party member, along with granting each the role of leader in turn, helps both game-play and planning.
Whichever method you use to delegate authority in your group, there's more to the choice of party leader than merely choosing one. Listen to the group so you can find the best path for you. Happy nominating!