Playing a slot you can pretty much determine how much money you want to wage. Usually, the minimum is € 0.01 per pay line, and you can even determine how many pay lines you want to play. To always play all possible pay lines by default, one simply uses Bet Max.
Using Bet Max entails you playing the maximum amount of pay lines. You of course still need to pay the wager per pay line at the level pre-set by you. It does not mean you wage the maximum per pay line; it means you play all possible pay lines.
Using Bet Max, and thereby using all possible pay lines has a few advantages. Namely, the more pay lines you play, the bigger the chance that you win at least one. You, as a player, of course, need to give it your all to enlarge your chances of winning; using the maximum bet function and thus playing all pay lines is a great start.
Furthermore, it is always advisable to play all pay lines when playing a jackpot slot. Namely, the more pay lines you play, the higher your chances of winning a jackpot!
There is really only one disadvantage to Bet Max; it costs more money to play more pay lines. As stated above, all pay lines most be paid for, hence the increased costs. Playing fewer pay lines is less expensive but comes at a cost regarding your chances of winning.
If you really want to increase your chances of winning big prizes and even jackpots, use Bet Max.