A card’s rarity gives collectors a general idea of the odds of receiving that card type when opening a series pack. The higher the rarity, the lower your odds of receiving one in a pack. Higher rarities such as extremely rare or chase cards are intrinsically more valuable to collectors because there are less of them made available to collect. In other words, lower supply = higher value.
The rarity of a card matters most when trading with other collectors as they are a good indication of the card's value to the collecting community. You will find the most success when offering balanced trades based on card rarities and series type: unlimited vs. limited edition series.
Please note: For limited edition series, rarities are only relative to other cards within the same series because each limited edition series has a different, finite number of total cards available for collection. For unlimited edition series, rarities are the same across all sets.