Actually, there's also a good reason for buying all the stuffs earlier.

1) Budget.. sometimes, u indeed need the item and got budget for it. And it happen there's a sale which usually lasts for a short time.. By buying earlier, you get sales prices instead of normal price.

2) If  you got your stuff ready... the contractor can do better judgement of how to renovate your house, considering the size of the the stuff that you are planning to put at certain locations. Eg : you're doing table top and want to have space for Fridge next to it. If you got the fridge ready, the contractor can estimate how long the table top should be.. Same thing with hood & hob.. The item must exist first before they can make a hole to fit your hob..

3) You can bring it your lamp/fan, and try putting it at your favourite location. You can see whether it looks beautiful at that location or not. That way, you can advise the wiremen on how and where to do the wiring.. If you buy the stuff once the renovation complete, you walk in into lighting house and saw a nice light and attracted to it but since you've never imagine about such thing before, there's no power point to fix your light.. If you buy it, you need to call the wiremen to hack your house again.  So, it's also good to do some shopping first while renovation in progress, so, there won't be unexpected extra stuff that buy after that..
All above are true from my own experience, not from my own theory