Why play every day? And, what are Loot Tickets?

You should play at least once, if not twice or more, every day.

For 2 good reasons.  First, you should participate in your Alliance as much as possible.  Attack titans and join Alliance Wars.  You may not do very much damage in those contests to start with, but you'll do more and more as you level up AND participating qualifies you to share in the rewards when those contests are finished.  Your fellow Alliance members will greatly appreciate it - Alliances with many members who are not actively participating have a hard time winning their combat contests.

Secondly, using your World Energy earns experience, no matter if you're advancing on the Map or doing Quests.  Gaining Experience Points advances your Player Level (not to be confused with leveling up your hero Teams).  As you raise your Player Level your reservoir for World Energy increases and you can play for longer stretches.  This becomes necessary in the Season 2 Map, where it can cost a lot of World Energy to play the later sections.

So to maximize the rate your Player Level is increasing, you should drain your World Energy flask as often as possible.  It regenerates at a rate of 1 point per 10 minutes.  

However, you will reach a point in advancing on the Map where your heroes are not yet strong enough to beat the monsters in the next contest.  So how to gain experience then?  Loot Tickets!

Loot Tickets allow you to quickly collect the rewards AGAIN for a Map level you have already finished, without having to slog through battling the monsters again.  This takes the drudgery out of draining your World Energy flask repeatedly each day.  Loot Tickets can be won as loot from some Quests and Alliance War Chests.  But that is a relatively rare reward.  They can also be purchased in the Shop, but the best way to get them is to keep an eye out for Special sales where you can purchase both gems and other items at the best combined price.  Sometimes these sales include large numbers of Loot Tickets with your gems purchase.

Finally, you can also get a few Loot Tickets each day by purchasing a VIP Pass, which I strongly recommend for reasons you can see in another post here.