Couldn't resist chiming in on this one.
Problems with today's fish (my personal hypothesis based on my re-entry into the hobby in 2011). In the early 1980's I owned an aquarium cleaning and setup business, had no issues with fish then and never quarantined anything. Only disease issue then was ick.
1) genetic issues due to inbreeding depression, an issue in some species.
2) fish "back in the day" may have taken weeks to go from the wild to the local fish shop, with any weaklings dying from illness during the very stressful shipping process (pre "Fed-Ex"). This left only the very strongest specimens for sale!
3) DISEASE - recirculated central filter systems moving disease through multiple shop tanks, massive aquaculture and rapid shipping get sick fish to stores and into our homes. Why should the big-box shops care? I have personally spent >$20 in meds to save a $2.50 fish I purchased. Not a bad business model if you sell sick fish and then the meds to treat them! Went through literally 15 juvenile angels from a big box store before I started medicating the QT tank automatically, then lost zero after that.
More fish, better prices, but at a cost...
Regarding test kits. Helpful, but not a requirement unless you need to adjust the water for particular species and/or are impatient to see if your tank has cycled fully!