Strikeouts as a percentage of plate appearances — for hitters, a contact-frequency measure; for pitchers, a swing-and-miss measure.
Strikeout rate is strikeouts divided by plate appearances. For a hitter it reflects how often an at-bat ends without contact; for a pitcher it reflects bat-missing ability. It is distinct from whiff rate, which is per swing rather than per plate appearance.
K% stabilizes relatively quickly, which makes it a reliable input early in a season compared with outcome stats that need much larger samples.
On the pitching side, K% and the opposing lineup's K% are the backbone of strikeout-prop research; on the hitting side, a low K% supports hits and total-bases volume.
K% is strikeouts per plate appearance — an outcome. Whiff rate is misses per swing — a process input. A pitcher can have a high whiff rate without a high K% if hitters foul off two-strike pitches or put balls in play.