Overwatch Guide – Can Winter Lootboxes Be Opened After the Winter Event?

What you see is what you get! It's just an opportunity for players in the match to commend each other on good gameplay. :)
_{@PlayOverwatch on Twitter (May 30, 2016)}

Cards can reach "Epic" or "Legendary" status at 5 or 10 votes respectively. The only 'reward' is a unique voice line from the hero on the card. There's a full list of commendation voice lines on the Overwatch Wiki.

In other words, commendation votes are like clapping. They do not have any long-term tangible impact.

Turrets: these seem pretty strait forward but can actually be used in a variety of ways. Obviously the most strait forward is to cover choke points. This is extremely effective IF you can place them in such a way, that is inconvenient for the enemy to have to destroy. For example, if you place turrets above the enemy, on to your side of the wall, when they walk through they are forced to look up to shoot it. They are using that time to look up to shoot the turret and not your team. That's a minor victory in my book. How I like to use the turrets however is a bit different, and I'll tell you why. Symmetra gets notifications when her turrets are attacking, or when they get destroyed. I generally place 3 at the choke point if I'm defending, and I'll place the remaining three around the area where sneak attacks are common. This allows me to spot the attack well ahead of time and warn my team mates. Do not under estimate this, it's extremely useful. Also they slow, so there is that. I should also note that the turrets can be placed ON payloads, so when attacking you get mobile turrets, which can be nice. (I don't cover turret placement itself very well here, that's a topic in and of itself. however there are good tips from other commenters below)

Source: https://www.reddit.com/r/OverwatchUniversity/comments/4lmr4u/why_symmetra_is_amazing_and_how_to_use_her_kit/

Don't hesitate to read the Reddit post, you have many more information about the rest of Symmetra's kit and the synergy between her abilities !