I think it's a way of showing up the contrast between Robin and Charlotte. Charlotte thought nothing of sabotaging Strike's birthdays and throwing his presents out the window if he had displeased her in anyway; Robin is able to exercise self-control and think of other people's pleasure, even if she is unhappy herself. She works hard at working out what people like - she's generous and thoughtful, unlike Charlotte who was demanding and selfish, and showed her generous side only when it suited her own goals.
Also Robin comes from the sort of family where they care about these things, and Strike didn't really have the sort of stability where family members make an effort to show your importance to them. He can make green dress sort of big gestures, but that routine relationship maintenance has never been modelled to him.
Birthdays and celebrations and gifts (and funerals and flowers) are really important in Troubled Blood because they mark where the characters stand in relationship to each other. When Strike sees a family celebrating an 80th birthday at a pub, just for a second he thought Robin had organised a surprise party, which he would have hated, but also it would have been a mark of her affection for him, and he wonders if anyone will be with him on his 80th - it's the very beginning of him thinking whether he does really want to be alone for the rest of his life. When Robin sends flowers to Joan's funeral, it's a reminder that he does have a family of sorts back in London. Morris' gifts of chocolates (and Strike's of flowers) don't take into account what the person would really like to receive and so they are not relationship building gifts, just gifts of convenience.
When Strike takes Robin to choose perfume, he's getting her something she really wants - and also I think facilitating her evolving into her own person. She's rejected the perfume Matthew likes, and won't choose the perfumes Sarah and Charlotte wear. She chooses a couple of perfumes in Liberty that she likes - so she's in control of who she is becoming - but then seeks Strike's opinion - he is involved in and approves her transformation, and that's important to her.
Four paragraphs on presents. I really need to get a life. 