According to Wikipedia, the $105$th cyclotomic polynomial is interesting because $105$ is the lowest integer that is the product of three distinct odd prime numbers and this polynomial is the first cyclotomic polynomial that has a coefficient greater than 1.
$$ \begin{align} \Phi_{105}(x) =\; & x^{48} + x^{47} + x^{46} - x^{43} - x^{42} - 2 x^{41} - x^{40} - x^{39} + x^{36} + x^{35} + x^{34} \\ + \;& x^{33} + x^{32} + x^{31} - x^{28} - x^{26} - \phantom{2}x^{24} - x^{22} - x^{20} + x^{17} + x^{16} + x^{15} \\ + \;& x^{14} + x^{13} + x^{12} - x^{9\phantom{8}} - x^{8\phantom{8}} - 2 x^{7\phantom{8}} - x^{6\phantom{8}} - x^{5\phantom{8}} + x^{2\phantom{8}} + x^{\phantom{16}} + 1 \end{align} $$
I don't see why this property of this particular polynomial makes it interesting, can anyone elaborate on this interesting coincidence?
I would say it is interesting or at least relevant to know that not all cyclotomic polynomials have coefficients in $\{-1,0,1\}$, which one might naively believe given a a list of the first few. To give the example $105$ is thus interesting and relevant; whether the exact phrasing is optimal to convey this is a matter of style.
To insist on the fact that $105$ is a product of three odd primes, is also pertinent. As it hints at the fact that numbers that are product of at most two prime powers have coefficients $\pm 1, 0$ only. And, also for $n= 2^k p^n q^m$ this is still true. Yet it fails for the first example that violates these conditions.
Continuing from there, there are various questions one can ask, for example one might try to characterize those $n$ for which one only has coefficients $\pm 1, 0$. For this problem see for example Cyclotomic polynomials with coefficients $0,\pm 1$ on MO.