Relays??????

im an Organic Irish grower this is my third grow at the minute, 5 venus flytrap and 3 urban poison feminised from Nirvana, currently under some cfls which i used before but my 600 watt hps is on the way, just one question though - seeing as a 600 watt uses 5 or 6 amps and a plug can take 13 as can my timer why is it reccomended to get a relay? the start up power cant be more than double the usual amps surely? thanks in advance.
about irish street weed - youd actually be better off with legal shit 9/10 times

startup surge can be 4-5 times the rated power draw for a few miliseconds. over time, it may arcweld the contacts closed.
when I used cheap timers,I just replaced them every other grow.I now use a pricey digital so I use a relay.

so its preferable but not necessary?