Confusion; What (really) is an Interposer Board used for?

I have several PowerEdge 1950/2950/2900 systems that  I work with and am confused as to what the interposer boards on SOME of the SATA drives actually do. Through experimentation I was able to find out the following:

1: A SATA drive with an interposer board (IB) shows up as SATA in the raid configuration utility (CTRL-R on a PERC 5i). I expected this, but verified just to try to be thorough. 

2: A SATA drive with an IB can be used in a RAID5 array with similar SATA drives that do NOT have an IB. This works as a member drive as well as a hotspare.

3: A SATA drive CANNOT be used as a member or hot spare (with or without an IB) in a SAS array and vice-versa. They CAN be used on the same backplane in a different array. I expected this as well, but try to touch all of the bases...

4: A SATA drive without an IB will fit into the SAS position on SAS or SAS/SATA drive caddies and work perfectly well. (They will also fit into and work in PEx8x0 SCSI caddies in a PEx9x0 system if you remove the blocking screw/blue plastic plug depending on the version of SCSI caddy being modified...)

5: I am able to install 8 SATA drives with no IBs into the 8-drive backplane on the PE2900s without any recognizable issues.

So what exactly are these really cool looking interposer boards used for? About the only thing I can think of is that they may be useful in larger drive arrays to buffer/amplify the signal between the drive and backplane similar to how registered/buffered RAM works, but I can't find any confirmation.

I do see some discussion saying that the SAS/SATA caddies are too long and the IB is used as an extension, but in my experience the only reason that the second set of SATA specific screw holes exists is to allow use of the IB. If I don't use an IB I can install a SATA drive in the SAS position and all is well with the world! (I also see where some people state that SAS and SATA use the same connector set... This isn't completely accurate as SATA drives generally uses separate data and power connectors that have a gap between them while SAS drives have the gap filled so as to require a single connector. SATA drives can use a SAS cable or backplane, but a SAS drive will not work with separate SATA cables without a messy modification.)

So anyway, what exactly does the interposer board do?

Thanks in advance for any explanation or clarification that you can offer.