I resisted buying one when I ran a lawn service in the 1970's and never saw the need for one and wasted time not covered by the customer. I had always trimmed close with the mower and called it good. I had a few customers tell me that I needed to purchase one so I purchased a corded trimmer but only used it on their property. I donated the trimmer to my parents when I moved to Idaho to work on a PhD in 1982.
During COVID, our Grandniece could not find work during the summer so we invited her to help with our yardwork. I had been using a walk behind and sickle bar mower to mow around the the property. I figured she could use the walk behind mower for most areas. It work great except for under the electric fence where the walk behind mower left grass at each post. Hand trimming proved time consuming so I needed a line trimmer. My corded trimmer was a Black and Decker so I knew them for quality and they worked for homeowners.
In 2020, I purchased BLACK+DECKER 20V MAX Cordless Battery Powered String Trimmer Kit with a 3 Ah battery & charger from my local True Value hardware store. It was on sale for less than $100 and my first 20V powered tool. The trimmer/edger has 2 speeds. The line is advanced when the trimmer is turned off or the easy feed button pushed so it is not a bump feed system. Best of all it met my criteria of light weight (10 lbs) and not require gas mixing.
Reviews indicated the battery life was short and a second battery was needed for most big trimming jobs. An additional 20v 3Ah battery was purchased from Amazon but when arrived found it was 1.5Ah. I needed to read the fine print better but it works and I was trying to keep the Grandniece busy.
The BLACK+DECKER 20V MAX Cordless Battery Powered String Trimmer has replaced my sickle bar mower when trimming under the electric fence and around the mail box. It will trim a 3x450 ft swath before the 3Ah battery and operator runs out of energy. Fence trimming is a 2 day operation when I do not use the sickle bar mower first. I seldom use the edge function of the trimmer. The few times where the trimmer was used as an edger I found it eats line fasts and ultimately bought a blade edger for edging. It is ideal for trimming around objects after mowing as close to the object as possible. I have seen a lot of lawn care professional trim first then mow on Youtube but often wonder why use a trimmer over their big mower. Trimmers are for trimming and not mowing. No it is not a "Big Boy Toy" where you trim before mowing but works for trimming small areas after mowing.
If grass is tall or has thick stems consider mowing or using hedge trimmer.
If lots of trimming consider using a plant growth regulator to chemically mow the grass to keep it short instead of trimming. Embark Turf and Ornamental Growth Regulator is a good one.
The spindle hole on spools of extra string are often too tight for the 1/2" shaft so use a 17/32" drill to enlarge.
Line diameter is 0.065 " and I use 2 to 3 - 30 ft spools per year.