You don't need one after you have used one to learn how to grow.
But then if you change nutes
or strains
you may have better result by using a meter,
even if you are experienced.
Following a nute calculator
and not being sure if you made all the correct measurements
can be fatal.
If you have a meter
you can see if you measured correctly
and you can see of the plant is eating
when the tds falls.
You will learn more about your plants
if you learn to grow with a meter.
All the nute calculators are very general guidelines
and not specific to weed, or lettuce, or tomatoes
and you will do better if you learn to use a meter
to feed your plants the lowest tds possible.
Remember that roots uptake water and nutes by osmosis,
and if your nutes are very salty, high tds,
then osmosis is harder to achieve.
Your plants are happiest
and your buds will be stickier
on a lean diet.
.