Because Aristotle

"Aristotle developed four logical methods to help people argue their way through complex issues ..." -- S. Crowley, Ancient Rhetorics for Contemporary Students.  Pearson, 2004) (Source of quote here.)

"In a logical proof, the premises may or may not all be true, the conclusion is a consequence of the premise-set, and, therefore, the conclusion may or may not be true. What we can say in the case of a logical proof is that it is logically impossible for the conclusion to be false unless at least one of the premises is false." -- M.R. Cohen, An Introduction to Logic.  Hackett, 1993) (Source: same.)

Proof No. 1:
1.  I have to go to the store for OTC medicine.
2.  French onion dip is at the store.
Therefore, since I'm going there anyway, I might as well get French onion dip.

Quod erat demonstrandum.

Proof No. 2:
1.  Shortly I will be in the possession of French onion dip.
2.  What am I, an animal?  Am I going to eat it with my fingers?  Have I no dignity?
Therefore, I also need to get some chips.
 
Proof No. 3:
[Gotta go, bye!]