Why shouldn't you use maximum power all the time?

It's Illegal International treaty (ITU-T) limits radio transmitters. In the United States, most wifi transmitters operate "Class C". You are obligated to shutdown, if reasonably informed you cause interference. You are authorized to use no more power than the minimum required for reliable communication. Regulatory fines can exceed 80,000 USD/day.
Don't be a bad neighbor For every 3dBm increase (doubling) of power, you will be visible over twice the physical area, and thereby to twice as many neighbors. There are a limited number of channels, so if you are visible, you are unneccessarily reducing the time they can use the channel.
Don't waste battery Without wifi, a Raspberry pi draws about 1000mW to 2000mW. At full power, the builtin wifi adapter will drain the battery in about half the time, or may even overload and shutdown the USB socket supplying power.
Don't waste bandwidth Learning from history (problems with Citizens Band radio in the 1970s United States), IEEE 802.11 "power management" reduces bandwidth per associated client as more unassociated clients acknowledge beacon reception. Each unassociated client that hears you on the channel, slows all your associated connections. If 90% of your reception area clients are associted with other access points, your bit rate will be slow by a factor of ~3.

Like ripples from a fishing bob on the surcace of a lake, radio waves spread as approximate circles of descreasing strength until they blend unnoticeable with the lakes surface. The size of the circles range from meters to kilometers depending on transmitter power from a milliwatt to watts.

A reliable connection requires the signal be about two to ten times the background noise. If the network area is ~20m across (e.g. an apartment), The signal should fade into the background in the orange region, about 13m to 26m from the transmitter. In the blue region, the signal is sometimes corrupted by noise resulting in reduced speed. In the yellow region, the signal is strong enough that speed is unaffected by noise. In the red region, the signal is so strong that it blocks other access points. If the green region is the edge of the network area, then everyone in the network area connects with optimal speed and reliability, those outside the orange area are unaware, and some in the blue and orange area may suffer reduced speed.