Why is it so important to first analyze measurement models and then structural models?

I got an answer but I think I did something wrong because my answer seems quite astronomical in pricing.

#5

the soil biogeochemistry in Californian salt marshes wished to explore how two of the most common plant species, pickleweed and pacific cordgrass, affected the accumulation of nitrogen in the soil. They measured the concentration of ammonium, a molecular form of organic nitrogen, in the soil, and measured or manipulated the density of pickleweed and cordgrass. Manipulations of ½ meter square plots (20 plots per manipulation) included removing all cordgrass (cordgrass removal), removing all pickleweed (pickleweed removal), or mixed (the control - nothing removed, thus mixed plots included a mixture of natural pickleweed and cordgrass). Boxplots of ammonium concentrations in each of the three manipulation treatments. Letters indicate differences among means as determined by a Tukey posthoc test following ANOVA. (b) Overall relationship between cordgrass stem density and soil ammonium content (includes data from all plot treatments). The solid black line and the gray shading represent the…