A lot of people watch the conventions, according to Nielsen, it was 33 million this time. The Presidential candidates nomination acceptance speeches draw the biggest television audiences of the entire election, except for the debates.
After a party's convention, their presidential candidate gains a few points in the polls. Political Scientists refer to it as the "convention bump." I would guess that most of the people who are part of that shift are undecideds who would probably have ended up voting that way to begin with, but I can't back that up with data. I also don't know how consistently people who start polling toward a candidate following their party's convention actually support that candidate by election day. It may very well be that they are just excited by all the pomp and circumstance, and that excitement could wear off by November.
In any case, the convention bumps tend to be fairly small and equal for both parties.