RMSE arises from what is probably the most important model in statistics, linear regression. A linear regression model is fit with least squares, which means minimizing the mean square error (MSE) for the sample. Take the square root of MSE, so that it's on the same scale as the data and hence easier to interpret, and you get RMSE.
If that just makes you wonder why we fit linear regression models with least squares, here's how Wikipedia puts it:
The OLS [ordinary least squares] estimator is consistent when the regressors are exogenous and there is no perfect multicollinearity, and optimal in the class of linear unbiased estimators when the errors are homoscedastic and serially uncorrelated. Under these conditions, the method of OLS provides minimum-variance mean-unbiased estimation when the errors have finite variances. Under the additional assumption that the errors be normally distributed, OLS is the maximum likelihood estimator.
That's a lot of nice properties.