Water warmed in a kettle has much more even temperature in all points, which affects the brewing process. Generally, the more even the temperature is, the more consistent and rich is your brew.
I would consider microwave boiling as a makeshift method to produce a mediocre result when you need it anyway, not as a daily driver.
I'm asking this from a place of genuine ignorance: how does the evenness of the heat distribution matter when microwaving a pure liquid? I'm familiar with the microwave's uneven heating qualities. I'm sure we've all bit into food that is scalding hot on the surface and still lukewarm at best in its interior. However, I've always presumed that is a product of microwaving a heterogenous, predominantly solid substance.
So, sure, the microwave applies heat unevenly to the water. But wouldn't the tiny little bits of water which get "over" heated simply diffuse their excess thermal energy into the rest of the homogenous volume in very short order? Furthermore,wouldn't an uneven heat distribution in a mug of water simply lead to convection currents flowing from hot to cold, therefore promoting a relatively even distribution?
The overheated particles will rapidly move upwards, which will lead to relatively even distribution in a layer, but uneven between heights.
In fact, in a large microwaved mug the difference between top and bottom can be as much as 6°C/11°F.
Using a kettle mitigates it for the most part, as it is the bottom that gets continuously heated, and the top is then naturally heated by the vertical currents of hot water, leading to a more even distribution.
But at that point, isn't it easier to just buy a kettle? It doesn't require such manipulations, costs next to nothing and allows you to rapidly boil up to 1,5-2L (0,4-0,5 gal) of water for all your needs.
There's a good reason most of the (Western, at least, dk about other places) world uses them and considers them a basic piece of kitchenware.
How does a kettle warm the water more evenly but a microwave doesn’t? When a kettle has it’s heating element only at the bottom but a microwave blasts the entire mass of water with energy because it sits on a rotating plate.
In my experience you won't actually boil water in the microwave because it takes an eternity so you end up with tea in "warm" water instead. Or apparently some people also put the tea bag in the microwave ¯\_(ツ)_/¯
It may appear that way if it was unevenly heated, causing pockets of boiling water surrounded by comparatively cool water. This would make it look like it's boiling, but then, when mixed, it is then much cooler than if heated by a kettle that relies on convection to mix the water.
The microwaves will heat your water more evenly than a kettle.
Liquids have this amazing property, that if you heat them , they auto-stir just by themselves.
(But personally, I'm uneasy about microwaving a tea bag with paper on one end, or worse, a staple. There's probably no problem at all, but it doesn't feel that way.)