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Animals have centrioles to form spindle fibers during prophase. The centriole divides cells. Plants do not have them because they have microtubules instead; they do not need centrioles. Plants are capable of forming a circular loop of microtubules around the future plane of division prior to prophase called the preprophase band, rather than centrosome. Basically- plants don't need it. They have a different way of cell division.
The centrosome, also called the "microtubule organizing center", is an area in the cell where microtubles are produced. Within an animal cell centrosome there is a pair of small organelles, the centrioles, each made up of a ring of nine groups of microtubules. There are three fused microtubules in each group. The two centrioles are arranged such that one is perpendicular to the other.
During animal cell division, the centrosome divides and the centrioles replicate (make new copies). The result is two centrosomes, each with its own pair of centrioles. The two centrosomes move to opposite ends of the nucleus, and from each centrosome, microtubules grow into a "spindle" which is responsible for separating replicated chromosomes into the two daughter cells.
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Its just the way mother nature has made them. Simple. In animal cells too, centrioles are not absolutely essential for microtubules to organize. If we remove them experimentally, even in that case the cellular division occurs uninhibited.