Why would there be two RNAV waypoints co-located with a VOR? Similar to this question but more so!
The Enroute H-5 chart shows two RNAV waypoints over Spinner VORTAC (SPI): SOHOW and USIRE. Neither waypoint appears on the Low IFR charts, nor on the VFR charts.


Enroute H-5 as of Feb. 25, 2021, via Skyvector

Searching the online Form 8260-2 database I see that Spinner is used for many different instrument procedures:

ROCKY SID off of Indianapolis
LAKES SID off of Kansas City
NATCA SID off of St Louis (an RNAV procedure!)
Several Victor airways and J-routes
Several instrument approaches to SPI and other nearby airports (including use as an IAF or feeder fix for RNAV approaches!)
MOTIF STAR into Chicago/Gary
Bradford STAR into Chicago
Braymer STAR into Kansas City
RIVRS STAR into St Louis
Vandalia STAR into St Louis

Some of these procedures only use Spinner to define a waypoint on the route, but in other procedures Spinner is actually flown over.
SOHOW does not have a Form 8260-2 listed in the database, but from a general search I notice it is used in:

LUCIT STAR into Gary

USIRE is used in:

KOLTS STAR into Indianapolis

Comparing the location of the three waypoints:




Waypoint
Latitude
Longitude




Spinner
39 50 23.04N
089 40 39.85W


SOHOW
39 50 25.37N
089 40 37.43W


USIRE
39 50 23.04N
089 40 39.06W





SOHOW is ≈300 feet northeast of Spinner
USIRE is ≈062 feet due east of Spinner
USIRE is ≈270 feet southwest of SOHOW


So the question is: Why do SOHOW and USIRE exist? And in particular why do they both exist? I can think of three reasons:

There is a rule saying that RNAV procedures must only use RNAV waypoints.
Counterargument: Spinner is a waypoint on the NATCA (RNAV) SID.
Spinner will be decommissioned soon, and the RNAV waypoints will be used instead. Neither waypoint is exactly on top of Spinner, but perhaps it doesn't need to be; the LBNON waypoint will replace Lebanon VOR/DME soon and those do not have the same exact coordinates.
Counterargument: Spinner is used in several VOR approaches and to define fixes on other instrument procedures; also I haven't heard of any plans to decommission it.
The specific configuration of the LUCIT and KOLTS arrival procedures necessitate the distinction between two points 270 feet away from each other (less than 0.05 nautical miles).
Counterargument: Really?
Your Questions: Why do SOHOW and USIRE exist? And in particular why do they both exist?
Why do they exist: SOHOW and USIRE are published Waypoints on RNAV STARs into Gary, Ind., and Indianapolis, Ind., respectively. They exist and are published because they are part of the procedure being flown (as assigned by ATC).
Why do they both exist: Even though they are in extremely close proximity to each other in space, they are not physically located anywhere. The fact that both of these RNAV Waypoints are only &quot;points in space&quot; (geographic coordinates in a computer) reflects the value and flexibility of Area Navigation.  Perhaps both STARs (to Gary and Indianapolis) could have used either SOHOW or USIRE, but there is no physical reason to do this.  Also, since each of the Waypoints (SOHOW and USIRE) are part of different STARs going to different airports, there is less opportunity for confusion as to which procedure is being flown.
As a note, you mention in reason number 1 of your post (your Counterargument) that Spinner Vortac is a waypoint on the NATCA (RNAV) SID.  Spinner is actually a &quot;transition&quot; on the NATCA SID, which technically ends at NATCA.  See image below.
Good questions.