Reveal is a keyword action. By keywording it, the Sixth Edition rules change clarified that there was a difference between a player looking at hidden information (usually something in a player's hand) and that player revealing it which meant that all players saw it. This distinction isn't important in a two-player game but matters very much when three or more players are involved. Also, the game would later care about things being revealed. 
From the Comprehensive Rules (Rivals of Ixalan (January 19, 2018))
- 701.14. Reveal
- 701.14a To reveal a card, show that card to all players for a brief time. If an effect causes a card to be revealed, it remains revealed for as long as necessary to complete the parts of the effect that card is relevant to. If the cost to cast a spell or activate an ability includes revealing a card, the card remains revealed from the time the spell or ability is announced until the time it leaves the stack.
- 701.14b Revealing a card doesn’t cause it to leave the zone it’s in.
- 701.14c If cards in a player’s library are shuffled or otherwise reordered, any revealed cards that are reordered stop being revealed and become new objects.
- 701.14d Some effects instruct a player to look at one or more cards. Looking at a card follows the same rules as revealing a card, except that the card is shown only to the specified player.