Make A Fist Before Striking
This proverb teaches us to attack from strong groups, not weak ones. Therefore, one should sometimes reinforce before attacking. Another way of looking at this: the reinforcing move has a value in itself (e.g. taking territory). If there is an attack as follow-up, it qualifies as a double-purpose move and can be sente.
Here, aims at the marked invasion (besides taking the corner). If the invasion is played before Black 1, White can still dive into the corner.
See also: Leaning