A lot of people like sleeping under blankets, and especially if you’re sleeping with someone else, maintaining a comfortable temperature both while going to sleep and staying asleep can be hard.
Small space heaters, window air conditioners, fans pointed at one side of the bed, lots of layered sheets and blankets, including some only on one half of the bed can all help with this. Most modern thermostats let you set multiple temperatures automatically, so its not hard to program it to adjust to awake and going-to-sleep comfortable temperatures.
Theres more expensive tech out there as well, so if you have the money and inclination look into it (cooling/heating beds and matress pads/pillows, beds with separate side settings, and lots of others).
The cost can be a consideration for some people, and that certainly shouldn’t be ignored; however, a lot of people tend to not properly compare the value of improved sleep to the low to moderate cost of buying and running a window A/C unit or space heater, especially some of the more efficient ones.
Don’t write off something as “too expensive” or too hard, at least do a very quick “how much does this actually cost, is better sleep worth whatever else you’d spend that money on” analysis, you’ll be surprised.