This answer is largely a rendition of Sridhar's comment into lambda calculus. A strong monad $T$ has the following introduction and elimination rules in the lambda calculus.
$$ \frac{\Gamma \vdash e : A} {\Gamma \vdash \mathrm{val}(e) : T(A)} $$ $$ \frac{\Gamma \vdash e : T(A) \qquad \Gamma, x : A \vdash e' : T(B)} {\Gamma \vdash \mathrm{let\;val}(x) = e \;\mathrm{in}\; e' : T(B)} $$
You may recognize these rules as typing a variant of the do-notation in Haskell.
Strength is needed to interpret the elimination rule, since the context $\Gamma$ is available in both premises of the elimination rule. Taking $\sigma : \Gamma \times T(A) \to T(\Gamma \times A)$, we can calculate:
$$ \begin{array}{lcl} e & : & \Gamma \to T(A) \\\ e' & : & \Gamma \times A \to T(B) \\\ T(e') & : & T(\Gamma \times A) \to T^2(B) \\\ T(e'); \mu & : & T(\Gamma \times A) \to T(B) \\\ \langle id; e\rangle & : & \Gamma \to \Gamma \times T(A)\\\ \langle id; e\rangle; \sigma & : & \Gamma \to T(\Gamma \times A)\\\ \langle id; e\rangle; \sigma; T(e'); \mu & : & \Gamma \to T(B)\\\ \end{array} $$
Without the strength $\sigma$, we could not use the context $\Gamma$ in $e'$. That is, we would get introduction and elimination forms:
$$ \frac{\Gamma \vdash e : A} {\Gamma \vdash \mathrm{val}(e) : \Diamond A} $$ $$ \frac{\Gamma \vdash e : \Diamond A \qquad x : A \vdash e' : \Diamond B} {\Gamma \vdash \mathrm{let\;val}(x) = e \;\mathrm{in}\; e' : \Diamond B} $$
I changed the notation from $T$ to $\Diamond$, since this is actually the possibility modality of S4 modal logic! These modalities arise in applications like functional languages for distributed programming --- e.g., see Murphy et al's 2004 LICS paper "A Symmetric Modal Lambda Calculus for Distributed Computing".
