League of Legends resets ranked once a year, at the start of the new ranked year. The reset is soft: your rank is not wiped to zero, and most players start the new year about one tier below where they finished. The later seasons within the same year normally carry your rank over without another reset. Before the new rank is shown, every player has to play five placement games, and those five matches are what a placement boost on Boosting24 is about.
Placements are not only for returning players. A fresh account that has just reached summoner level 30 and owns at least twenty champions also plays its placement games before it receives a first rank. The ladder it is placed on has ten tiers, from Iron, Bronze, Silver, Gold, Platinum, Emerald and Diamond, each with four divisions, up to Master, Grandmaster and Challenger. Where the account lands after those five games decides how long the climb to your goal will be.
During placements the rank is hidden and the game relies heavily on your hidden matchmaking rating, the MMR. Your starting division depends on the results of these games, how strong your opponents were and your rank from the previous year, so each single result carries real weight. That is why five wins or five losses in a row can make such a large difference to the division you see afterwards. A strong run sets you up for the whole year, and a weak one leaves you with a long climb back.
Many players therefore treat placements as the most important games of the year and prefer not to leave them to chance. Some order a placement boost because they have been away and are out of practice, others because they simply do not have the time for a careful start. Whatever the reason, the goal is the same: the best possible starting division for the rank the account really deserves. Five games are also a small, clearly defined order, which makes it easy to plan and quick to finish.