Reach exists because flying exists. A creature with reach blocks fliers as if it were one itself, and that is the entire job: stand on the ground, swat what tries to go over the top. At a table where one player has invested in an evasive clock and the other has not, reach is the line between losing to a 2/2 with wings on turn six and chump-blocking it until something better shows up. It tends to live on green creatures, often spiders, often with defensive stat lines that read as overcosted until the flier shows up and they become exactly the card you wanted.
The keyword reshapes a matchup more than a board. It does nothing the turn no flier attacks, which is most turns against most decks, and it offers no offensive upside on its own. What it does is collapse an entire axis of evasion for the cost of one card, which is why it shows up so often in sideboards: a single reach body can turn a lopsided air race into a stalled ground game where the green deck's other advantages reassert themselves.
The honest weakness is that reach is purely reactive and purely narrow. Against a deck with no fliers it is a vanilla body, and against fliers with enough power to attack through the blocker it still loses the trade. Reach on a 1/1 protects nothing; reach on a 1/4 for two mana is a format pillar. The keyword is only ever as good as the toughness underneath it.