I think it is so the water from the defrost cycle has somewhere to run too. Rather than over a path, freezing and causing a potential hazard. My installer asked the question and will fit a pipe (heated) and run off water will flow into the nearby drain. No soak away required 

This is an extract:

4.2 Safer design of the condensate discharge

The product contains R290 refrigerant. In the event of a leak, escaping refrigerant may get into the ground via the condensate discharge. The refrigerant must not get into the waste­water system. For the ground installation, the condensate must be discharged via a downpipe into a gravel bed which is located in the frost­free area.

4.3 Safe execution of the condensate discharge,

The downpipe must flow into a sufficiently large gravel bed so that the condensate can trickle away freely. To prevent the condensate from freezing, the heating wire must be threaded into the downpipe via the condensate discharge tundish.
The downpipe must not be connected to an existing underground drain pipe that is connected to the waste­water system.

I assume it's because of the R290 refrigerant, which (courtesy of google) is propane.  But this is only in the event of a leak, it would normally be just be water that is discharged? 

But don't forget I'm just looking stuff up and don't have any experience of these matters!! 

@julianc, I guess that technically we have a soak away for water to run off into over gravel around the ASHP. To be fair, I've monitored that and it never discharges huge amount of water, but I suppose if it was on a pathway, it could freeze over and become dangerous.

@kev-m, that's actually really helpful and I never considered the refrigerant. My installers are here today, and I'll ask them about this.

@kev-m

Never been asked for a soakaway across a wide range of manufacturers over decades.

I have just checked the LG manuals for the Therma V and they are more concerned with fixing the unit to a secure base.

Most manufacturers require a concrete base with the unit mounted on anti vibe mounts with adequate drainage, given the small amount of moisture produced during defrost.

If in doubt, pipe it away.

I have some clarity about the soak away on the Vaillant units and it's to do with the R290 refrigerant which is propane (and therefore flammable). Vaillant are therefore very strict on their installation criteria with regards to drainage, soak aways and clearances around and above the pumps.

@kev-m

The best solution, of course, is not to have a leak in the first place!

This is why we insist on FGas registration to ensure that proper procedures are followed.

No gas should enter the system unless you are sure that the system holds a vacuum and passes pressure tests.

@frevillebabe Just had my Vaillant Aerothem Plus installed and as you say no condensate pipe has been provided, distilled water all over the patio.  I have just suggested to the installers that they supply a drain pipe on their snagging visit and I will dig the pit

@witchcraft The drain pan on the units should have a connection point, one or more, that allow connection of a drain pipe. The units require a strong base to stand on but provision needs to be made for drainage of condensate to a suitable point. It should be part of the site survey at the very least.

I attach a diagram of the LG Therma V base pan showing five drain connection options. they usually come with drain plugs installed to be removed as required.