Dogberry's motivations were driven by his want to be acknowledged by the town as a respectable citizen. However, because of his lack of understanding for most insults, he usually just ends up insulting himself in front of everyone. This makes people think he's weird and a fool. Furthermore, he's a prison guard, which is the modern day equivalent of a mall cop. Because of this nobody takes him seriously.
"If I were as
Dogberry's a man who, although he means well, doesn't really understand English too well which gets him in a lot of awkward situations. His men catch Borachio and help him bring to light who was really behind Hero's disgrace.
Impact on Plot
Dogberry has a pretty big role in moving the plot forward, even though he's not even introduced until half way through the play and has minimal screen time. If it weren't for Dogberry and his crew, nobody would have ever found out about Don Jon paying Borachio to wrongfully accuse Hero and her name would have never been cleared. Had Dogberry not been in the play, Hero's "death" would not have been in vein and Claudio would not have agreed to marry her after her death was revealed to be a scam.