That unilet is called a "C". It is usually inserted in a long conduit run, just in case the wire pull becomes difficult. It can also be used to get by the 3600 rule - no more than 3600 of bends between junction boxes, panels, cabinets, etc. But I never inserted a "C" for getting by the rule if I thought the wire pull would stress or stretch the wires.
The NEC saying what it will, my personal experience is it's bad practice to use unilets as splice boxes. The "T" unilet can be called a junction box because it is the junction of (3) conduits but there's just not enough room to safely make a splice. But splicing in a unilet is something I learned should be avoided.
There are many practices you learn in the field that are contrary to the NEC. That's why some municipalities edit or rewrite the code. Experience tells you sometimes the NEC shouldn't be gospel. Just my two cents.
BTW, unilets are considered fittings, not boxes. At least that's what we call them in good old Chicagoland.