What's the purpose of reserve lines? Why do new hires have to sit reserve? Is it more or less a probation period? I ask because I honestly don't know. Do you actually do any flying on reserve?

Reserve pilots are needed to fill flights when someone calls in sick or doesn't show up, a crew gets stranded due to wx or mx, or for any reason the company needs a pilot to fill a seat. And its not just FO's, the lowest seniority captains sit reserve also...

As far as if you fly or not, depends on your company and depends on your base.

very true, at COLGAN I never flew on reserve, well unless it was 1 hour before my last day of reserve ended, then they would call me to ferry a SLAAB around th Northeast, then of course I got to fly it back the next morning on my day off, after I slept in a hanger all night man it was fun, at Express you fly a lot, and they actually give you more than an hours notice.....................

'Cause we're bottom of the barrel, buddy- bottom of the barrel.

Everybody has reserves, to cover planned or unplanned outages. How much you fly on reserve depends on who you're flying for, your base and eqipment. Last year sitting reserve in DAY for PSA I logged one day of reserve flying for .8 hours in Feb, five days for 25.5 hours in March, and eight days for 29.6 hours in April. In the CLT base over the summer I averaged about 12 days and 55 hours. I jumped ship to Eagle and sitting reserve in LGA I flew 13 days for 60 hours last month. (I would've flown more but I sat in ATL for 3 days waiting on MX to fix an airplane) This month I've already flown 10 days and logged 46 hours. We're so short I expect I'll fly every day I'm on duty this month & break guarantee.
I sat reserve at PSA for 18 months, and there was no end in sight. Eagle want to hire 600 more pilots this year, so holding a line by summer is not an unreasonable expectation. I have a buddy at Continental who was one of the first hired two years ago, he sat reserve in EWR one month, then held a line. Transfered to IAH in his 4th month and held a line by his 6th.
USAir guys with 18 years are sitting reserve. (Because 17 year USAir guys are furloghed.)

Sitting reserve can be a good deal - you're getting paid guarantee and may stay home the whole month. Commuting to sit reserve is a form of punisment that should be restricted to child abusers and corrupt politicians.

reserves in a company can vary greatly. ive been on reserve for a month and a half in SJU with eagle and averaged 8 hours a week or so. one guy beneath me had put in 25 hours in three days and had another turn and an overnight when i last saw him today. theres no real way of knowing how much flying you might do its just luck of the draw.