Just as a preemptive warning, there are a lot of bitter people on these forums that will call residency/fellowship a scam or unnecessary. Be sure to keep that in mind that while there are legitimate critiques out there, most of the time it's bitterness (at least on these forums).
That said, residency is only useful if it's paired with a high functioning pharmacist. Residencies don't magically make people into upstanding clinicians or even people. If you were a dumb a-hole who interviews well going into residency, you're going to be a dumb a-hole who interviews well after residency.
For me, it opened doors in that it allowed me to train and gain experience without an employer "taking a chance" on me right out of school. No one in their right mind today would hire an oncology pharmacist or a critical care pharmacist right off the bat, but hey, there I was in both of those settings during residency soaking it up and getting training. I got to make mistakes, be stupid, and all the things a first year pharmacist gets to do, but what that turned me into was a decently serviceable pharmacist and my new employer could say to him or herself, "Hey that guy was nice and he looks residency trained, so he got all the stupid 1st year mistakes done already, let's hire him!"
Of note, it's virtually impossible to get an inpatient clinical job in the San Francisco Bay Area without completion of a PGY-1. Virtually, not completely, and we did just hire a non-residency trained pharmacist (one of our in-house interns) to a hard-to-fill position.
Also remember it's job and geography dependent. If you want to be a critical care pharmacist at Ronald Reagan/UCLA in Los Angeles, it's pretty much going to be PGY-2 or equivalent experience.... or bust. If you want to be a general pharmacist at a 25 bed critical access hospital in Clear Lake, CA, and that's your dream job....residency is unnecessary.
I had a clinical job lined up on the east coast and I opted to do a residency on the west coast. Why did I do it? I wanted to be back in California, and residency was my fast track back (I would have stayed on 3-4 years at that first job, taken BCPS, and jumped west after that). My income differential my 1st year practicing after PGY-1 vs. what I would have made at my first job was +$42,000/yr, it's now +$75,000/yr based on estimated hourly rates put forth by SDN members for inpatient jobs in the mid-Atlantic region. My residency paid a bit over $50k/yr at the time (much more now), and so I pegged my break even point at roughly 2.5 years (which was 2.5 years ago, oddly enough)
So it worked for me, financially and personally. It doesn't work for everyone that way.