Research quests, and more specifically the field research tasks that make up those quests, are an excellent way to earn rewards and catch legendary Pokemon in Pokemon Go.
First introduced in March 2018, Pokemon Go research allows you to earn various rewards for completing Pokemon Go related tasks. These are broken down into field and special research.
The latter requires you to complete a series of fixed tasks, which are more complicated than field research tasks, in order to earn a rare reward. For example, the "A Mythical Discovery" special research will give you an encounter with Mew.
These special research tasks tend to have eight or nine stages, with three tasks making up each of them. Each task completion earns XP, while completing each stage provides rewards such as stardust and poke balls.
Field research tasks are far simpler and random, and are issued in poke stops. Upon completion they provide rewards, usually in the form of a Pokemon encounter.
There is also the mechanic of getting a stamp for every day you complete a field research task. Earning seven of these (they don't have to be in a row) earns you an encounter with one of several legendary Pokemon. This is called a research breakthrough.
The reward for completing what can be seven simple field research tasks is well worth earning. Obtaining a field research stamp every day means you can encounter a legendary Pokemon once a week.
Consequently, it is worth managing your field research tasks so you can get that stamp every single day.
Tasks range from making x amount of nice/great throws, to catching x amount of a specific Pokemon type. They are simple tasks which can be completed by simply playing Pokemon Go as normal.
When you complete a field research task in Pokemon Go, the completed task will sit there waiting for you to claim the reward. If you already have a stamp on that day, don't claim until the following day. That way you can guarantee a stamp the following day without doing any work.
The only downside to this is it takes up one of your allocated field research task slots. However, this is a small price to pay for a guaranteed stamp.
Some field research tasks are harder than others. Making three excellent throws in a row or winning five gym battles may prove difficult and time consuming.
If these are tasks you don't want to waste time on, you can simply delete them using the trash can icon next to the task. Spinning a poke stop will give you a new task in its place.
Furthermore, you can try to remember which poke stop gives a specific field research task as this remains constant.
It is worth noting that while some tasks are a lot more difficult than others, they do provide greater rewards for completing them. For example, hatching five eggs will give you an encounter with a Chansey. Elsewhere, making three excellent throws in a row will earn you an encounter with the sought after Larvitar.
If you are astute with your field research tasks and redeem them on a daily basis, you can really make the most out of the legendary Pokemon on offer.
The March and April field research breakthrough rewards will be the following legendary Pokemon: Raikou, Entei, Suicune, Lugia, Ho-oh, Regice, Registeel and Regirock.