PDT): Obtain new promo cards such as Poipole and Stufful and obtain event shop tickets by completing missions
They copy themselves for each spell cast before it, so it's possible to cast enough spells to burn all your opponents with Grapeshot, or mill them all with Brain Freeze if you're able to cast a ton of spells in one turn (made easier with the Hullbreaker Horror combo)
Yes, you can delete it
It introduced some mechanics and concepts that seem integral to the game today, including the use of counters on permanents, the creation of creature tokens, and creature types matter effects
If you are looking to cut back even more, the only card that's really replaceable is Pull from Tomorrow, which would save about $15
If you have no intention of continuing the subscription, you can simply activate the trial, claim all the benefits and then cancel 24 hours before the trial is over and if you do that, you will not be charged