Prices changing so frequently is simply the result of an adjustment between demand and supply in the market. Demand and supply change all the time, because market venues allow them to change very frequently, and so does the prices. This does not mean that the fundamentals of the underlying assets are also changing, this is just the way market process (what it perceives as) new information.
Say the price of a stock is $x_0$ and I'm about to buy 1,000 units of that stock. Effectively, I will buy those 1,000 units at a price $x_1>x_0$ because at a given point in time demand increases so the price adjusts. However, if there is no new information, it's likely that somebody else would want to sell 1,000 units after that, and then the price would adjust downward at $x_2<x_1$. Note that we do not necessarily have $x_1=x_0$ though. It depends on much more factors that in my simple example, but the idea is there.
The price changing a lot, also means that you, as an investor, are able to buy/sell at any point in time. If, as you said, prices were updated once a day, that means that you would be able to make a transaction only once a day. Actually this exists on some market, and in general the price is set through an auction process. But obviously, if possible you want to be able to buy/sell at the moment you take the decision, and in general you would dislike to wait until the daily batch processed.
Regarding, the high frequency that the most liquid markets have reached now, this is the result of the competition between market venues. Indeed, in current days, there are often several places where you can buy/sell a particular stock for instance. Since market venues are seeking to attract the largest possible amount of transaction on their market, for liquidity reasons, this results in market competing on the frequency they offer to market participants.
Actually, this is a bit more complicated, let me just sketch up some words. Algorithmic traders are the type of market participants that provide a lot of liquidity because their are able to buy/sell extremely fast, there are the new market makers in a sense. The people who set up those algorithms (large banks mostly) are also seeking for profit. They know that high-frequency will make them more efficient than classic human investor, so they are looking for the fastest possible way to make transaction on a market. And market platforms, wanted to attract those liquidity traders (the algorithmic traders), have increased (a lot) the frequency of their market. This is not necessarily bad, actually if you buy/sell something on a exchange-market, it's likely that your counterparty will be an algorithm. The question of whether it's efficient or not, especially in time of crisis, is more opinion-based and there are some research on that issue.