Hey ambar33n, thanks for asking and it is a good question that a lot of people struggle with. It is true that currently the huge price swings in the value of Bitcoin is a problem for adoption in trade. Compared to many of the large stable (but slow but steady inflating) fiat currencies (for example dollar and euro) the volume of Bitcoin exchange trade is still relatively low, which means not much money is needed to push the price massively up or down. A lot more money is needed to push the price up or down for the dollar. The expectation is that once the market capitalization and daily trading volume of Bitcoin go into the trillions, it will be much harder to move the price so much as Bitcoin currently does and it should become much more stable.
Another thing to look at is that for most Western countries that have relatively stable governments and economies that are reasonably well managed, not all usecases for cryptocurrency are directly visible yet, but for people living in poor countries, that have no easy access to the traditional finance or banking, and their government truly mismanages their local fiat currency (Venezuela, Zimbabwe are 2 extremes in terms of hyperinflation), Bitcoin is actually a better alternative: their local currency inflates hundreds or thousands procents each year, while looking at longer term Bitcoin's value has only gone up with hundreds procents still, despite huge intermediate price swings. Those people much more prefer a store of value and medium of exchange that is volatile but longer term goes up, instead of only going straight down like their local fiat.
By the way, due to the inflationary nature of most of the larger fiat currencies, long term they systematically lose value due to steady inflation, while Bitcoin is a non-inflationary currency which at least theoretically long term should outperform fiat currencies despite intermediary price swings. For perspective: the dollar has lost around 33% of its value (purchasing power) between 2000 and 2017 and even MORE THAN 50% of its value in the last 30 years; and that with the guarantee it will NEVER go up EVER! There is a funny infographic going around on internet that visualizes this issue:
Another value proposition for Bitcoin is that it replaces cash money which is systematically being abolished in many countries. This puts way too much power in the hands of banks and governments and since power corrupts, many believe that it can lead to all sorts of undesired side effects like negative interest rates, unlimited censorship and no way to opt out of the system if the system turns against any- or everyone. At least Bitcoin provides a system that gives an alternative to opt out of the traditional system. It provides people with a leverage against banks and governments so they can't abuse their power after all cash has been disappeared from public life.
Having said all that, still short term the huge price volatility is huge barrier for mainstream user and merchant adoption. However the more Bitcoin grows in value and exchange trading volume and the more merchants worldwide will accept Bitcoin (or even better price their products/services in Bitcoin), the stabler it should become and the more useful it becomes. Currently most of the users and holders are people that are true believers in the technology, the benefits it brings to the table and that bitcoin/cryptocurrency will play a major role in finance and economy in the future. These people also are less worried about short term price swings.
I hope this pretty long answer gives you some insights, to come back to your direct question if it is a loss for you if you'd buy at 20k and next day it is 15k. It is true that at that point you personally have a PAPER loss, however as long as you don't sell it is not a REALIZED loss in fiat currency value AND you still have the same amount of Bitcoin "units". As long as you have good reason to believe that the value will grow over time at least higher than your purchase price it doesn't have to become a loss. The other side of the coin is that in a situation where the value has gone up from the purchase price, you are at a PAPER gain in fiat currency, which is only a real gain if you sell and lock in that profit OR buy products/services for it with the Bitcoin itself (which is ultimately the goal: spending bitcoin for products and services instead of or alongside of fiat currencies).
(btw we are Bitcoin for Beginners, so it is alright if you don't understand everything yet; we try to provide access to the best resources and have a trusted platform for expanding knowledge and education in the crypto space)
