Once you get familiar with slots you will understand how they work and that each payline needs a coin in order to be active and therefore give you the chance to win.
The coin payline basically is the idea that you bet at least one coin on each pay line in a slots game. The size of the coin that you use is up to you and also depends on how much money you have available for your bankroll. Obviously the higher your coin size, the greater chance you have of winning more money as t he winnings are based on multiples of your coin size. But, that does not mean that lower coin sizes don’t bring in winnings, they do, just at a lower rate.
If you have a limited bankroll you might not want to spend all your coins in one go and on a 20 pay line slots machine you can choose how many paylines you bet on at each spin. In other words if you want to make your game last longer when you have a limited bankroll you can choose to only bet on 5 or so paylines per spin. This does reduce your chances of winning but at the same time can increase your playing time. It is a totally personal decision how you choose to spend your bankroll and in which way but just remember that for maximum payouts, each line should have a coin bet on it.