What are the uses of the timer in the shop?
You can buy "timed practice." Afterwards, when you click the "practice" link beside your tree (on the desktop version) you're given the option of practicing with a timer. You must answer quickly, because the clock is ticking. A correct answer gives you a little extra time on the timer.
It's just a way to make practice more like a game and less like drudgery.
The timer is to time yourself when you do a practice test. Use it if you are training to translate faster.
Could you relocate this discussion in the “Troubleshooting" forum? Which is the place meant for such discussion ("How Duo works?", "What is/How works this or that feature/thing?", etc.). Thx!
To relocate it (*), edit your original post then select said forum in the drop-down menu on top-left of the edition area (see here for details).
(*) /!\ please, don’t create a new one /!\
Have a look to this short guide to help you knowing where to post your (future) discussions on Duolingo’s forums.
A little warning ahead:
If you buy it from the lingot store, you cannot revert your choice.
Some people do not like it, and Duolingo basically asks you on every exercise with that multiple-choice selection what it shall start: normal vs timed
The code for "timed practice" also has been tweaked by staff and now it got much harder, even if you are on the L1-L3 crown levels.
Sometimes it is really tough to get any XPs.