[Brightcolours]

I'm slightly wondering whether those 3 Tamron lenses are really so interesting. These are all f/2.8s after all.
The 20mm is, IMHO, the most useful of three. The other two ...

You are correct in that they are only f2.8, and as such would also be covered by f2.8 standard zooms.

However, there is only the Sony E 16-55mm f2.8 G, which is pricy (€1300) and pretty heavy (~500 grams). And that lens only achieves 1:5 magnification, where these Tamron lenses' USP is their 1:2 macro ability.

That makes 3 arguments for the Tamron f2.8 primes.

1. 1:2 max. magnification ability
   
2. comparatively low price
   
3. quite a bit more compact and light than the standard zoom alternative
   

One of my favourite images I have taken on APS-C was with 35mm at 1:2, so I can see the interest of these lenses for the niche audience that likes wider angles close ups upto 1:2. And of course, they do also do infinity stuff, besides the 1:2 ability.

Taken with a 35mm at 1:2, vertical images stitched., shot with Canon APS-C.
The FF equivalent to 35mm on Canon APS-C is 56mm. 

Also shooting with a 55mm 1:2 (max. magnification) lens on FF I can appreciate these Tamron 1:2 APS-C offerings.... The choice of FOV matters for the outcome, also at close up and macro ranges.


Taken on FF with a Nikkor 55mm f3.5 micro.
So if my platform would be Sony APS-C, I would be quite interested in this Tamron trio, due to their close up ability.