The Bad Honestly, not much
You can tap them to create a Disenchant, Braingeyser, Terroir, Shivan Dragon, Regrowth, or Black Lotus, and then you can cast it, but you have to pay the cost for it
However, its by no means necessary especially because there are plenty of resources online to identify and assess every card
Half of that money will go toward Extra Life, benefitting Seattle Children's Hospital, Wizards of the Coast said
While life gain is only any use against the agro decks for the most part (as in you wouldn't feel compelled to play any life gain in your average deck if the field was all control and combo) it is often worth including as an alternative to lowering your curve and therefore average power level
You cant increase the size of your sideboard with the size of your deck, so as you add more cards to your deck, you reduce the impact your sideboard cards can have