While usually relegated to the comic relief of One Piece, Usopp has slowly found his place among the Straw Hats crew. He went from village liar and coward to sniper for one of the most infamous pirate crews in the world. While Usopp still maintains some of his cowardly habits, it would be unfair to say he doesn't deserve a place by Luffy's side. Not only are his marksman skills top notch, but a recent addition to his arsenal has saved the group more than once.
During One Piece's two year time-skip, Usopp trained with Heracles on the use of Pop Greens. These little balls are seeds of rapid-growing plants that, once fired, grow into various types of hostile plants from the Boin Archipelago. Usopp has taken these seeds and fires them like bullets from his slingshot Kuro Kabuto, a trick he learned from Heracles. It was while training that Usopp learned how to use the seeds and get his body into top physical condition. He even added an upgrade to Nami's Climate Baton that allows her to also launch seeds in a similar manner to his slingshot.
Each Pop Green will grow under certain conditions. One type may grow when it hits the target, another while still in the air, or when triggered like a trap. To ensure he never runs out of ammo, Usopp has built himself a garden on the Thousand Sunny where he cultivates these plants and their seeds. Each plant has a specific use, like the Humandrake plant which creates little humanoid mandrake sprouts that latch onto an enemy target. Many of the plants have offensive uses in One Piece, while others have defensive or passive effects. The color of the seed denotes which plant and type of effect is inside.
As one of the less powerful members of the group, it's fitting that Usopp would make use of something that utilizes his own strengths. His marksmanship is unparalleled, so being able to use plant seeds that grow almost immediately is one of the more creative solutions to using Usopp effectively. While he's not as physically strong as Sanji, Zoro, Luffy, or Jinbe, he's intelligent and tactical. The seeds allow Usopp to attack from distance, but also in closer combat. All he'd have to do is drop a seed that would allow him enough time to withdraw or break away.
Usopp will probably never become as strong as his friends in One Piece, but that doesn't make him weak. It just means he has to find other ways of being effective in combat. While he acts as the long-range fighter, it's important for him to have a defensive option when opponents close in. The Pop Greens give Usopp more versatility as a fighter. Given his penchant for tinkering, he could eventually create new weapons with them for himself and his crew mates, especially with Franky's help. He's already upgraded Nami's weapon to utilize the Pop Greens, so it's entirely possible that he could further expand on this.