In effect they replaced nothing, because they were introduced to provide rolling stock for the new - I expect they said "enhanced" - Midland Main Line service that MML (The TOC) provided. Roughly speaking, there was a slow and a fast every half hour and they connected cross platform at Leicester: either a Nottingham slow connecting with a Sheffield fast, or a Derby slow with a Nottingham fast.
Well, that was "enhancement" wasn't it? Up until then, under late BR and MML, you'd had a mish-mash of 1 TPH sometimes express, sometimes some intermediate stops to Sheffield and ditto 1 TPH Nottinghams.
What the 170s did (or tried to do - except they didn't have the right capactiy to do it properly) was go back to the BR system introduced in 1969 IIRC and then double it.
To explain (for the beneift of the OP), from that time - with Cl 45s and LHCS of course - BR ran a semi-fast to Leicester, where it waited for an express that left St Pancras 30-35 minutes later to overtake it. The semi fast fed into the express cross-platform, and vice versa.
The express then left for Sheffield, alternating between Derby one hour, Nottingham the next. The semi-fast then followed, stopping at Loughborough and then terminating at Derby or Nottingham - whichever was not served by the express.
This gave a one hour express service (or nearly) to both Nottingham, Derby and Loughborough - if you didn't mind changing on alternate hours.
When HSTs were introduced in 1982, originaly on the expresses only, their faster timings meant the semi-fast had to depart just 10 minutes later from St Pancras (ie 50 minutes in front of the next HST) to make it to Leicester in time. I suspect this resulted in quite frequent delays all round if the semi-fast was late for whatever reason. (Maybe someone can confirm, I wasn't using the trains by then.)
Whatever, when the line went all HSTs (bar peak hour commuter services) in about 1984, BR scrapped the previous system, and ran two HSTs per hour, to Sheffield going via Derby, one to Nottingham. ie it no longer sent Sheffields via Nottingham.
This provided 2 'usable' TPH to Leicester (good for some!) - but with irregular stopping patterns at intermediate MML stations (bad for Luton/Bedford/Kettering etc).
MML inherited this system, and as you say, then introduced the 170s, which provided 2 TPH to the intermediate stops, and freeing up 2 TPH HSTs to run fast to Leicester, and on to Sheffield/Nottingham.
So this sort of doubled up the earlier BR system - except, as mentioned, the 170s didn't have enough capacity for the job! MML then introduced extra centre carriages, and later the 222s to solve the capacity problems.
(The explanation of the BR system is a bit simplified, but I think it contains the essence of the service at that time.)