$\mathcal{N} \ge 2$ Supersymmetry massive supermultiplets

In Bertolinis SUSY notes [https://people.sissa.it/~bertmat/susycourse.pdf] we have defined: $$\{Q^I_\alpha,\bar{Q}_\dot{\beta}^J\}=2m\delta_{\alpha\dot{\beta}}\delta^{IJ}\tag{3.24}$$ $$\{Q^I_\alpha,Q^J_\beta\}=\epsilon_{\alpha\beta}Z^{IJ}$$ $$\{\bar{Q}_{I\dot{\alpha}},\bar{Q}_{J\dot{\beta}}\}=\epsilon_{\dot{\alpha}\dot{\beta}}\bar{Z}_{IJ}$$

Also we define: $$a_\alpha=\frac{1}{\sqrt{2m}}Q_\alpha\;\;\;\;\;a_\dot{\alpha}^\dagger=\frac{1}{\sqrt{2m}}\bar{Q}_\dot{\alpha}$$ Lastly, the central charges $$Z^{IJ}$$ can be written in the form $$Z^{IJ}= \left(\matrix{0 & Z_1 \\-Z_1 & 0\\&&0&Z_2\\&&-Z_2&0\\&&&&\ddots\\&&&&&0&Z_{\mathcal{N/2}}\\&&&&& -Z_{\mathcal{N}/2}&0\\} \right)\tag{3.28}$$

(Where the charges are non-zero only for even $$\mathcal{N}$$)

From these we define the following:$$a^r_\alpha=\frac{1}{\sqrt{2}}\left(Q_\alpha^{2r-1}+\epsilon_{\alpha\beta}(Q_\beta^{2r})^\dagger\right)$$ $$b^r_\alpha=\frac{1}{\sqrt{2}}\left(Q_\alpha^{2r-1}-\epsilon_{\alpha\beta}(Q_\beta^{2r})^\dagger\right)$$ where $$r= 1,\dots,\mathcal{N}/2$$

These equations satisfy the oscillator algebra: $$\{a^r_\alpha,(a^s_\beta)^\dagger\}=(2m+Z_r)\delta_{rs}\delta_{\alpha\beta}$$ $$\{b^r_\alpha,(b^s_\beta)^\dagger\}=(2m-Z_r)\delta_{rs}\delta_{\alpha\beta}$$

How does one "see" that we need to define those equations for $$a^r_\alpha$$,$$b^r_\alpha$$?

Since you have a nice basis of harmonic oscillator esque creation/annihilation operators, you can define a supermultiplet by postulating a vacuum state $$|s\rangle$$ annihilated by $$a,b$$, so that every state in the supermultiplet is given by hitting $$|s\rangle$$ with creation operators $$a^\dagger,b^\dagger$$. Since the $$a^\dagger$$ and $$b^\dagger$$ are all fermionic the procedure will terminate at a finite number of steps, leading to a finite-dimensional supermultiplet.
You get different supermultiplets by postulating different $$|s\rangle$$s. You can label them by Poincare casimirs like helicity, mass squared etc. and central charge