Why alignment is needed after installing springs
 

Hope you pros can chime in. Thanks

It's common sense. When you lower a car, it adds natural camber which affects the toe. Install the springs and wait two weeks to do an alignment.

^^ That's pretty much it OP. You're supposed to wait a few days to let your suspension components settle, but I just get the alignment done right away. In my experience, the difference is negligible.

You want to get your camber and toe back into acceptable settings to achieve even tire wear. If you don't you'll have to fork out more money to replace worn down tires more often than normal. Also having bad toe settings is dangerous.

I neglected an alignment on my 04 G35 Coupe and it ended up costing me in the long run. My front tires were only wearing down on about 2" of width instead of the whole tire width. I hydroplaned into a concrete barrier at 50mph because I wasn't driving completely on maximum tire width (grip and traction). Repairs were $6000 compared to an $80 alignment. I did come out with a 370Z in the end though =)

Also, if you're lowering your car, you may need to purchase some camber arms. From what I heard on the forums you will only need a rear camber arm when lowering on springs. Coilovers will require both front and rear camber arms. I bought SPC rear camber for mine and did not need toe bolts to get within acceptable alignment specs.

My lesson learned for not buying camber kits and getting an alignment:

As already mentioned above. I noticed a distinct handling difference (i.e. car was skiddish as hell!!) at speed after installing the springs prior to the realignment. Car darted slightly left and right slightly, most likely due to the more aggressive camber AND the toe being out of spec. Toe out of spec is scary!

The realignment cost me @ $120 and was well worth it.

P.S. I also added rear adjustable camber arms and toe bolts before getting it realigned. I'm sure that also added to the misalignment.