You have an investment fund that prices using a single price - fairly common in Europe. They invest in highly liquid securities, so the bid/ask represent a good view of the price you would receive/pay to trade the underlying portfolio.
Why would you use the last trade price to value the fund to know what to charge people to buy or sell shares in your fund?
Why is a personal portfolio different?
The last trade price is not a good indicator for a security performance over time - particularly if it is fairly illiquid and has a large spread.
The last traded price of a security on any given day may be closer to the bid or offer price depending on the particular trade reported and hence will fluctuate. These fluctuations do not necessarily correlate with the change in value of a listed security, hence provide a less consistent representation of its movement in the market.