Double threat ladder-maker
When Black plays rather than connecting solidly, it gives White the option of and to set up two ladders (White at a and White at b). This is a typical crawling fight tactic.
Of course in this case Black may be content to exchange with Black c, White b now.
Another example comes up in the tenuki line of the tsukehiki joseki (see 3-4 point high approach, inside contact, tenuki variation).
Here after the / exchange depends on a ladder. More exactly White would now like to play hane at a.
There is discussion of follow up moves of this joseki variation at 34PointHighApproachInsideContactTenukiVariation.
This is from BQM 15.
In this case Black is probably happy to exchange the stones for the (the reason being that Black earlier made a joseki mistake).
What is said on BQM 15 is this.
One should add a little more: here must be answered by White, so that Black has an option of a alternative ladder also. Depending on the precise position in the lower right, this might be important.
Black might have a chance to push once more at a: reading required.