>>533577558
Lottery events are events where there is a material lottery that gives a large amount of experience, skill gems, materials, QP. They are the best time to get stuff you need to level all your servants. the difference between them and a regular event is that regular events usually only have a shop (and maybe a points ladder) with a limited amount of rewards, while lotteries will give their rewards infinitely so long as you have AP to use.

That said, you don't NEED to save apples for them, it's just a good thing to do. The game gives apples fairly often and depending on where your current account is at you may either not need materials that badly or not need. Don't be afraid to use some apples if you just want to power through some story or farming EXP for something you need.

The best way to tell if you'd benefit from farming a lottery would probably be to check if you have a lot of servants with skills at Lv. 1 to 5 that you want to get to 6 or higher. If there's a lot, a lottery will be useful to you. Skill gems are more difficult to farm generally, while the other things (EXP, QP etc.) can be farmed at any time on any day at your convenience.