Amp is plugged to the wall, why use a conditioner?

Depends on the conditioner and the amp(s). Also, before doing anything else, I'd have dedicated lines installed just for the stereo system. The usual recommendation is for 20 amp lines. My Tice Power Block III put the lie to the idea that conditioners compromise power amps -- surprising the heck out of the guy who built my amps-- and the more expensive PS Audio P10 makes them sound even better. (Two of my audiobuddies also have that particular Tice, by the way, and wouldn't be without it.)

There are a lot of GOOD reasons to use a line conditioner. Irrespective of audible effects, a good line conditioner like the Furman products filter a lot of line voltage transients that can damage your equipment. These transients over time can damage sensitive transistors and amplifiers. Not sure if tube equipment is as sensitive, by it only takes a nanosecond transient to destroy solid state devices.

Some products can provide power factor correction, and provide line capacitance that helps the transient response of power amplifiers. I use a Furman Power Factor Pro on all my equipment, particularly the power amps. It reduces line voltage sag when transients hit the power amp. I use another one for the low power stuff like CD players, A/D converters, preamps and the like to isolate them from AC power effects caused by the power amps.

Some line conditioners also can regulate the AC line to smooth out brown outs. I have used the Furman AR-15 which actually has a switched transforment that keeps the output at 110 VAC even if the line drops to 90 VAC or rises to 130 VAC. I have found these can be audible, however, so I only use them on computers and other electronics, not my audio components.

Thanks for the information.
I have a Krell HTS control amp, Sherbourn 7 channel power amp, Faroudja DV-1000 CD/DVD player, and a pair of Martin Logan Source speakers. Should I get a single conditioner for the amp and one for the rest? What models and brands will not limit current? Which are best to purify the signal? I hear the Shunyata 2 and the Richard Gray 400 are good for this. Any thoughts? How does the Monster 5100, 2000, or 7000 perform? Does anyone know the difference? Thanks Greg

I know for sure that my integrated amp sounds more closed in when it is plugged into a power conditioner vs. straight into my dedicated line. I keep it unplugged when it is not in use for a long time or if there are electrical storms.

Dopogue is right, it depends on the conditioner in question.The conditioner should have a amp(Kwa) rating adequate to match the needs of a given power amplifier. I use a BPT AC power isolation transformer that has a 20 amp capacity(same as my wall outlet-dedicated line).Dynamics of all of my amplifiers(3) are improved with the BPT not lessened.Every sound parameter improved without some compromise or trade off.This is true for every component in my system.
Regards,