A standard Carleman-type estimate is of the form
$$
\sum_{|\alpha|<m}{\tau^{2(m-|\alpha|-1)}\int{|D^{\alpha}u|^{2}e^{2\tau\phi}}dx}\leq K\int{|Pu|^{2}e^{2\tau\phi}dx},\quad u\in C_{0}^{\infty}
$$
where $\phi$ is some weight function.This formula turn to be very useful in the study of uniqueness of Cauchy problem,and many mathematicians have considered this(such as Calderon,Hormander,Kenig,Sogge,and Tataru...)
  
  For a first look at this inequality,I'm wondering whether the weight fuction makes a essential role,and what's the original idea of it?Is there some very simple but illuminated examples to show the the reasonableness of the Carleman estimates ?

 Well,the first example in my mind is the first order operator $P=D+ix$,then it's easy to see that $P^{*}=D-ix$,and 
$$
P^{*}P-I=PP^{*}+I=-\frac{d^2}{dx^2}+x^2
$$
which is the so-called harmonic oscillator,then we have 
$$
2\|u\|_{L^2}\leq \|Pu\|_{L^2},\quad u\in C_{0}^{\infty}
$$
But in this simple example,there is no need to put a weight function,anyhow, from the proof,I guess the decomposition $P=\frac{P+P^*}{2}+\frac{P-P^*}{2}$
may one of the general idea.