/etc/fstab if you have the
discard option for your SSD. Last time I checked, Manjaro does implement TRIM by default, but not this way.
They do not do the same thing. The timer does all the TRIM job once per week, and the
discard option does the TRIM job much more continuously (it is issued each time files get deleted).
The timer way is arguably the most preferred way among the Linux community, and arguably the one preferred on many distributions, but I could not tell why Manjaro choose
discard over following the trend.
It is useless to enable the timer if
discard option is used. If you end up not using
discard, well, enable the timer if you prefer using the timer.