I know that I'll have visited the map and I'll probably have tried every weapon…
Benefits I'm thinking about are like a new class, stronger weapons, emblems, badges…
If I finish the campaign mode, will I get some benefit when I play on-line
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Best Answer
If you mean will you unlock badges, better weapons for multiplayer then no. They all have to be unlocked by playing multiplayer only.
The only benefit you'll get is from more experience with the game. Like you say, you'll be more familiar with the weapons and slightly more familiar with the maps (although overall layout will be different).
I'd definitely recommend playing through single player first just to get your bearings with the game, but be advised multiplayer is a whole different ball game to single player, good luck!