To the downvoters: this is correct. Marking to market (looking at unrealized losses or gains, ie. the value of your portfolio before you sell) is useful if you

To the downvoters: this is correct. Marking to market (looking at unrealized losses or gains, ie. the value of your portfolio before you sell) is useful if you trade on margin, use this as a collateral (borrowed against it), or plan to sell soon. In all these cases, this is because you need to sell (directly or indirectly) and thus lock in whatever the current value is. But until you sell, you have no actual (realized) loss (or gain).