My battery charger has settings for 2-Amp, 15-Amp, and 40-Amp charge rates. My question is, why would anyone want to charge their battery at a 2-Amp rate? Obviously a 15-Amp or 40-Amp rate would be faster.

For that matter, why bother with 15-Amp when 40-Amp is available?

Inquiring minds want to know. ;)

high amp = faster, but harder on the battery.

high amp = faster, but harder on the battery.

Harder in what way?

Excessive charge rates can cause overheating, pressure buildup, outgassing.

Lead acid batteries usually have some kind of maximum charge rate in the OEM literature, but a good rule of thumb for charging is one tenth the amp-hour capacity of the battery.