When you warrants have vested you have the option, but not the obligation to subscribe for shares. Under which circumstances should you do that?
Generally, the price which you pay for a share using a warrant, the strike-price, is set at the price per share which is slightly higher than price per share at the time that the warrants are issued. As an example, if the price per share when you issue the warrants is 10 kr, you often set the strike price to 20 kr (for a discussion how to set the strike price, see this article).
If the price per share hits 20 kr or more, then the warrant is said to be in the money, that is, it makes financial sense to use the warrant to buy the share.
But since startups in general are not listed on a stock exchange, and shares rarely change hands. So how do you know what the value of the stock is? The most common way is to use the latest share issue. But if that happened a while ago, it is not obvious what the current value of the stock is.
In the end, as a warrant holder you must make a decision on whether or not you think it is worthwhile to invest. If for example the last share issue happened a year ago at 15 kr per share, and you now must decide whether to exercise warrants with a strike price of 20 kr per share, it may seem that the warrants are not in the money, since the latest share issue was at a lower price than the strike price. But if the company is doing well, it may very well be worthwhile to buy it anyway. If the company is not doing a new share issue and existing shareholders don’t want to sell, using the warrant may be the only way to buy share.