in all fairness, I don't think sticking money in premium bonds while you're deciding what to do with it - is a bad shout - you're not tied into anything, no withdrawal penalty, and if you've got the full £50k then with average luck you'll get the same or even higher amount of "winnings" as interest you'd earn in a standard savings account, with the added advantage of not paying tax on them, and of course the (remote) possibility of winning big, and, what is the big seller for most people = guaranteed security.
So for the short term, for example, if there's a gap between selling one house and buying another and you need somewhere to put your deposit/equity, or
in cases of inheritance where the money side is tied up with emotion and people might also be trying to sort probate and whatever else so just not have the capacity to maximise their finances on top of everything else, you could do a lot worse.
Same applies to people saving for something specific in the next year or two - a new car/move house/next year's holiday, or even just the elderly who realistically might not be around in 5-10 years to get the dividends from an investment.
Not to mention people who aren't good with finances or don't understand investing - not everyone is, and if you don't know what you're doing it's better to at least look after the money you've got rather than risk losing it all. Same with people who hate risk - peace of mind can be worth thousands.
It's like any type of advice on here - very few people are able to give fully advice on any subject; it's almost always influenced by what they would do in your position (or what they imagine they would do if they've never been in it), forgetting that they AREN'T you. It's easy to say "open a S&S ISA and build a tracking portfolio," but less easy to do when you don't even know what most of those words mean individually, let alone together.