@Catinthetwat
You'd think me odd OP. I take a bottle of 'still' mineral water everywhere, as I don't always want a hot drink, and when people give me a drink of water, it's almost always from the tap, and it's usually vile; warm - and tasting of lime. Also, if they offer a fizzy drink, it's often warm (and sometimes flat.)
I also take a pack of breakfast biscuits or a banana to peoples houses, because there is no guarantee they will have anything in that I want. Eg, several people I know are vegans and have only soy milk, and no normal biscuits - just vegan ones, and others I know have only tea and no coffee.
It's a bit annoying really because I keep vegan and veggie stuff in, and also tea bags (even though no-one in my house drinks tea.) And if I know I am having a vegan visitor, I get soy milk and vegan snacks in, but they never get non-vegan stuff in for me. I think it's quite thoughtless to be honest.
In addition to all this, I work odd hours, and have occasions where I can't get out to get anything to eat from 10am til 3pm, so I like to have a wee snackette in case I get peckish!
Have to say I don't find it odd that people take their own bottle of water or snacks. Especially if (like me) they are busy and work odd hours and don't know what time of day they will be able to eat properly.
Also, if someone does continue to take their own stuff to your house, you have to question if you are a good host, and are offering what your guests want. As I said, quite a few people I visit don't get in or offer what I like, even though I do it for them...