Why is it that Terminal Services employees need to work in pairs of one human and one Giftia?
Nothing they do as part of their jobs seems to strictly require the presence of specifically a human, nor specifically a Giftia. (And, I mean, given that humans and Giftias are virtually indistinguishable until near the end of the Giftia's life, what difference would it make, anyway?)
It is possible that this sort of pairing is for the preservation of human life. Once a Giftia becomes a Wanderer they start to act on instinct alone which sometime leads to violent confrontation.
In the pairs the human is referred to as the "spotter" and the Giftia is the "Marksman". The Giftia are as close to humans as SAI Corp has been able to make with the current technology but they are not human. If one reads Isla's character bio its states that she "often has difficulty understanding how social situations work" this minor flaw could potentially cause a team of two Giftia's to miss a Wanderer if it's in the beginning stages.
The human is because overall the human will have more experience with human/android interaction so they will be able to pick up on the subtle signs that could start showing when a Giftia's time is almost up.
Also having the Giftia there can provide a buffer in between the human and Wanderer, granted the Giftia is still limited whereas the limiting functions on the Wanderer are off. Terminal Services would probably rather have a Giftia injured in a retrieval rather than a human.
This is a potential reason for the pairing of the two but I could be wrong, not much is stated as to why the pair has to be like that. ref from Plastic Memories Wikia
Because being delicate in handling the relationship between other humans and Giftias is a part of their job
The terminal services is not an elite group of armed forces even though that would be more efficient/guaranteed of a way of doing things. This fact tells us that the company wants the more delicate approach, in an effort to respect peoples feelings during this difficult time. A best case scenario of a bitter-sweet parting of ways is hugely encouraged, and although it may not seem like it, that has everything to do with pairing the teams with a human and a Giftia.
When on various assignments, workers will need to deal with a large range of different relationships between people and Gifias, and having a (working) relationship of their own between human and Giftia means they will be able to relate, understand and empathise with each case and have a better understanding of how to handle the situation with respect.
It is also beneficial because of how people will react when the Terminal Service shows up. A group of humans, or a group of Giftias showing up would be more likely to lead to them becoming upset and thinking "you don't understand!". Of course the employees may not understand every situation, but having an openly displayed working relationship between Human and Giftia promotes the idea that they are understanding and caring and would mean people are less likely to react in this way when the pair show up.
Lastly, there are situations on the job where specifically a human may be required or specifically a Giftia. For example, if someone is hostile to all Giftias, then naturally having the capacity to send in just the human (or vice-versa) is very valuable, not to mention the different capabilities humans and Giftias have which are useful in different circumstances (as said in @SWard's answer) and this would not be possible if they were not paired purposefully.
