Complex sentence: Realistic filters can only approximate this, since an ideal sinc filter (aka "rectangular filter") has an infinite delay, but it is commonly found in conceptual demonstrations or proofs, such as the sampling theorem and the Nyquist-Shannon interpolation formula.
Split into simpler sentences: Realistic filters can only approximate a sinc filter (aka "rectangular filter"), which has infinite duration. But it is commonly found in conceptual demonstrations or proofs, such as the sampling theorem and the Nyquist-Shannon interpolation formula.