Why were the Specialist ranks (SP4, SP5, SP6) important? What purpose did they serve to the Army?

There are a lot of MOSs that require a lot of expensive training, skill, and experience. Those soldiers deserve increasing pay, just like their hard stripe counterparts, but they might not have the same leadership abilities. Dividing the ranks into specialist and sergeant lines allows the Army to have the technicians doing what they do best, and the sergeants doing what they do best. Unfortunately, somebody failed to keep that in mind back in 1985 and put them all in the same bag.

That's exactly right SFC Parker, however I'd have to disagree just slightly and actually supported the change. During the late 70's & early 80's the Army Re-aligned its career fields and discontinued many MOS's with quite a few being discontinued in technical fields. Example, my first 3 years in the Army was in MOS 52B (Prime Power Generator Operator & Mechanic) and loved that Job. In 1979 the MOS was discontinued and all of us were reclassified as 63B (Wheeled Vehicle Mechanic). I had no desire to be a Grease Monkey (most of us felt the same way) didn't have a lick of training in it, and Re-classified upon Re-Enlistment into another Career Field (71N, which later during the same re-alignment period became 81N). The end results were an imbalance and shortage of Sergeants & Staff Sergeants in the Technical MOS's resulting in the vast majority of Specialist 5's & 6's serving with direct Reports in Front Line Leadership positions. I for one was a Section Chief holding the rank of Specialist 5 (MOS 81N) as were the other 4 or 5 Specialist 5's in my organization and we had one Specialist Six with direct Reports (He was a Computer Programmer). All of us were lateralled by our Commanders up to the Hard Stripe level of Sgt. & SSG because we had Direct Reports. For the record, my MOS of 81N at that time did become hard Stripe at the E-6 level (Staff Sergeant), but there were even fewer Staff Sergeants which is why the Specialist 5’s was in direct leadership positions. The one lone Specialist 6 that we had also was a Sergeant First Class at the E7 level, but our organization didn’t have one (it just made no sense that his Rank wasn’t a Staff Sergeant at the E6 level). That was a very common scenario throughout the Army at the time. I knew of no Specialist 5’s during that time that didn’t have direct reports, although I’m sure there were probably some out there, but most were serving in Leadership positions as a Specialist 5 or lateralled up to Sergeant. That was one of the big major factors that went into retiring those ranks and making the permanent change to Sergeant & Staff Sergeant. I was in Brigade Headquarters at the time in Germany (1985), and Our Commander (a Colonel) and Sergeant Major held an official Ceremony. We had to switch back to our original Specialist 5 & 6 Rank Insignia for the Ceremony. They removed them and replaced them with the Hard Stripes making it official. It was actually a big ceremonious day for our Command, even though all of us were already wearing the Ranks lateralled up as "Acting's", so the Army just made permanent, what we were already doing in our minds.
Of all the Ranks I held & wore… Specialist 5 was my favorite. It was unique… recognized as both a NCO and a Technician… during my time in uniform. Although subordinate to Sergeant and Staff Sergeant they were treated and managed no lesser. They were pretty sharp looking Rank Insignia’s as well.

Why are Doctors, Dentists, Lawyers, veterinarians, Commissioned and most pilots are Warrants? Aren’t Doctors “Specialists”?

That we do sir. Just sad that the perception was there.

Yep, SUPER worker Bee. As a SP6 in the 115th MASH I enlisted 'too old' (33yo) to be an Officer, so as an ENLISTEE I "couldn't cut it" AND SERVED HONORABLY anyway.

In calibration (35H/94H), being a specialist instead of a hard striper was worn with pride. We are highly skilled technicians with very high aptitude test scores and abilities. We liked it that way. I very much disliked being converted back in 1985. We didn't need leadership skills. We had important equipment that need repair and calibration. instead, we were often given 'additional duties' that were a waste of our abilities.

I wouldn't say they "couldn't cut it," but that they served a different function than their NCO peers. IMO we should go back to that, because there are PLENTY of E-rank Soldiers whose skills don't lie in management and direct leadership roles but are still of value to the force, just like plenty of officers stumble in command roles but shine on staff.