The construction of the punchline of this joke is pure comedy 101. For some reason, I consider, it is important that the reader sees the punch of this kind of joke, that it's a sight gag, instead of reads it out of a word balloon. This lets the humor value of the drawing of Snoopy with a haircut assist the main joke (that of a dog, an unlikely competitor, beating Shermy to getting the first haircut of the summer).
It is important, I think, that the payoff be a sight gag, but I'm not exactly sure why. It might be because the rest of the strip is primarily verbal, so it needs the sight gag for variety. Or it might be because Shermy's reaction is spoken, and having two characters speak at the essential moment of comedy would be unwieldly. It might just have to do with that nebulous comedic concept, timing. Or maybe, if the punch moment of the strip were told instead shown, it'd seem arbitrary and forced.
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