So, I don't know the Softrock personally, but from your description, it's a switching mixer architecture.
Receiver side
What that means is that you mix not by multiplying the input signal with a single harmonic oscillation (a tone), but by switching it on and of.
Pre-Mixer HPF
Mathematically, this description is omitting one interesting detail:
The operation done with the input signal $r(t)$ of the downconcerter is not actually a multiplication with a perfect square wave, i.e. a wave that has the shape
$$l'(t) =A \cdot \text{sgn}(\sin(2\pi f t))$$
but is actually just an "on- and off" switching; so in the half-periods where $l'$ would be negative, it's actually zero. This leads to an actual LO function
$$\begin{align}
l(t) &= \max(l'(t), 0)\\
&=A \max( \text{sgn}(\sin(2\pi f t),0)\\
&=\frac12l'(t) +\frac A2
\end{align}$$
Whereas $l'$ has a time-average of zero, we can clearly see the DC offset of $\frac A2$ of $l(t)$!
Now, when multiplying
$$r(t) \cdot l(t) = \frac12 l'(t)r(t) + \frac A2 r(t)$$
we get the sub-$f$ content of $r(t)$ directly in baseband (to worsen it, amplified by $\frac A2$)!
So, with a switching down-mixer, you must have an input high-pass filter to avoid getting low-frequency input signal directly in your desired output band.
Post-Mixer HPF
On the output of the same mixer, you'd also want a DC-cancelling high pass filter – just because there's practically zero information in an arbitrary small bandwidth, but having a DC offset on your ADC will simply reduce your usable dynamic range. Anyway, that hpf is usually just the AC coupling you'd do anyways to get your signal onto the center of the $[0;V_\text{Full Scale}]$ range of the ADC.
Transmitter side
Pre-Mixer
You don't want DC offset from your DAC to reach the switching mixer – simply because that is typically simply a transistor, and imprinting a DC current would shift its operating point and possibly be detrimental to linearity.
Post-Mixer
The Mixer will introduce a DC current. You typically don't want that to reach your antenna, your HPA (same effect as for the switching transistor) or your baluns (which might be a DC short, anyway).