Do your clothes get drier when you use a dryer?
If you are comparing several items in terms of how dry they are, you would say that one is drier than the other and the third is the driest of them all.
I prefer a drier climate.
This is the driest summer on record.
The appliance that dries your clothes is, according to Merriam-Webster’s online dictionary, “usually” spelled dryer. But if you really want to spell it drier, that usage is not uncommon, especially outside of the U.S. (I don’t recommend using drys instead of dries, however—unless you are referring to those who were in favor of Prohibition!)
These fruits all have to be cut into pieces and then dried.
These are too dried out to use.
My well has dried up.
I’ll bring along some dried fruit as a snack.
Her home is decorated with dried flowers.
I can’t go until I have finished drying my hair.
Water your plant regularly to keep the soil from drying out.
Flier and flyer are similar; both words can refer either to one who flies the friendly skies or to the brochure or circular that you post to advertise your product or service, though the latter usage is “usually” spelled flyer.*
The flier returned from the mission a hero.
The team canvassed the city with flyers promoting the event.
In the U.S., airlines seem to prefer the y form for their frequent flyer programs—but the spelling used by journalists varies.
“The 14 Best Airline Frequent Flier Loyalty Programs” (Travel + Leisure)
“Survey: Majority of Americans Confused By Airline Frequent Flyer Programs” (Forbes)
In short, you are safe using either flier or flyer. (But note that the plural of the noun fly (e.g., the insect) is flies (not flys)!)
* According to Merriam-Webster, flier can mean either “a person or animal that flies” or “a piece of paper that has something printed on it (such as an advertisement or an announcement) and that is distributed to many people.”