Reliquary Tower is one of the most-played lands in Commander for good reason: it's a colorless land that costs you essentially nothing while solving a real problem. Plenty of decks draw aggressively and end up with eight or more cards in hand, only to discard during cleanup. The Tower lets you bank those resources.
It shines in draw-heavy decks: spellslinger/wheels, blue control, Nekusar-style group hellscape, or any deck running The Locust God, Niv-Mizzet, or big draw engines like Rhystic Study and Mystic Remora. If you're regularly holding excess cards, it's a free upgrade over a basic-tapped land — or even a colorless utility land.
When not to play it: in tight three-color or greedy mana bases where every untapped colored source matters, the colorless production can stutter your curve. Likewise, aggressive aristocrats or token decks that dump their hand quickly rarely benefit. If you're not consistently exceeding seven cards, the no-maximum clause is dead text, and you're just running a colorless tapland upside-down.
If multiple effects modify your hand size, apply them in timestamp order. For example, if you put Null Profusion (an enchantment that says your maximum hand size is two) onto the battlefield and then put Reliquary Tower onto the battlefield, you'll have no maximum hand size. However, if those permanents enter in the opposite order, your maximum hand size would be two.
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