How many times have you seen the explanation "I need incognito/private mode because I might be buying presents for my partner/wife/husband/whatever"? So many. And this reason is total bullshit. Why? Let me explain.
It's 2016. How do people use the Internet? From PCs, laptops, smartphones, fridges, you name it. If you are browsing for a gift on your phone, it's your phone. Nobody else should have access to it really, unless you want them to. So there's little need to have incognito mode because of shopping on that. You could also use apps for shopping, so even less so.
We use PCs. They're personal. If it's a shared one, I'm sure you have separate accounts set up on it. You haven't? Why the heck not? You don't want to personalize it the way that suits you? You're using a system where separate accounts are difficult to use? There still are those? So, again, no real need for incognito mode when you're browsing, since you can't see each other's history anyway.
You have computers at work usually. Again, you have your own account and the recipient of the present most likely doesn't have access to it. No real need for incognito mode for present browsing there either.
So do tell me why you need incognito mode because you want to surprise someone with a present. Really, do tell. I can't think of any.
And yes, I can think of other reasons to use incognito mode. I'm not saying it shouldn't exist. It has uses. But this particular reason always pops up and is totally pointless reason for that mode to exist.
However, if you are browsing some nauhgty sites on the Internet and want to hide that from your partner, ask yourself why do you have to. If your partner isn't ok with your Internet habits, there might be a problem that cannot be overcome by hiding things. That's never a good idea.