What are the advantages of incorporating covariates in the model?

Advantages of using a covariate are (1) a model with a higher goodness-of-fit, with a higher quality of the imputations and usually with lower standard errors of the indices and (2) TRIM tests whether the indices differ between covariate categories, which is often interesting to know. When you use a covariate in the model, TRIM calculates separate indices for each category, as well as an overall index based on the sum of the counts of all sites per year.