I personally ferment low, say around 62º on my ales and do a diacetyl rest with some yeasts (Ringwood as an example) and need to get that temp raised to the low/mid 70's. Having that heat source is the only way to get that to happen. Same with lagers where I need to really be able to adjust the temps from the 50's, up to the upper 60's/low 70's, then back down to lager temps.
Is it needed, no. But it's helpful when doing both lagers and ales. And to me helps maintain a steady temp. It won't dip or climb more than 1º of my target.
Kegtoe...it's not a manual adjustment. You just set your temps and variances on each input (I think they call it something like H1 and H2) and it takes care of the rest. Getting too cold...the freezer is kicked off. Getting too warm, the heater is kicked off. And you set that variance.