Why are topological ideas so important in arithmetic? For example, Wikipedia states that etale cohomology was "introduced by Grothendieck in order to prove the Weil conjectures". Why are cohomologies and other topological ideas so helpful in understanding arithmetic questions?
 A: If we think about Diophantine equations in general, the situation is "hopeless". That's a theorem. Nevertheless in number theory we want to study such equations, in special cases at least, so some ideas are required to sort out "Diophantine equation space" (DES) into parts that we might come to understand, and parts to leave alone. 
Classically this was done by "degree of an equation". This takes you a certain way, but not really far enough, as genus already shows. There may also be simultaneous equations, for example, and all the insights of algebraic geometry then come in. The slogan of "Diophantine geometry", that the geometry of a set of equations affects the diophantine properties, by now has much credibility. The part of DES where the geometry is well understood seems to correspond quite well to the part fruitful to study by current methods. (For experts, I'm sliding from integer to rational points here.)
"Topological methods of algebraic geometry" explains what happened in the 1950s and 1960s in that subject. From the 1970s, and really with much more pain, related methods have been used in Diophantine geometry. Those working in number theory have always been grateful for any general methods they can get with traction on DES.
A: Why are topological ideas so important in arithmetic? In some sense KConrad is of course spot on, but let me offer a completely different kind of answer.
Why are complex functions of one variable so important in arithmetic? (Zeta function, L-functions, Riemann hypothesis, Birch--Swinnerton-Dyer, modular forms, theta series, Eisenstein series...).
Why is geometry so important in arithmetic? (Faltings' theorem, applications of algebraic geometry, low-dimensional arithmetic of varieties (elliptic curves etc))
Why is K-theory so important in arithmetic? (Bloch-Kato, Voevodsky...)
Why is logic so important in arithmetic? (Julia Robinson, Matiyasevich, Ax-Kochen and then Hrusovski proving that "if it's true in char p for suff large p then it's true in char 0" in the context of some very deep statements)
Why is functional analysis so important in arithmetic? (L^2 functions on $\Gamma\backslash G$ with $G$ a semisimple Lie group being related to automorphic forms and hence to number theory via Langlands, with crucial analytic tools like the trace formula).
Why are dynamical systems so important in arithmetic? (3x+1 problem, work of Deninger, or of Lind/Ward and their school).
Here's the answer: it's because arithmetic is a very mature subject---it has been around literally thousands of years, and because it has been around so long, there is far more of a chance that someone will come along with an insight relating [insert arbitrary area of pure mathematics here] with arithmetic. So in some sense it's a historical fluke. If we were all born with continuum-many fingers which we could move only in real-analytic ways, and we didn't discover the positive integers until much later on, then arithmetic would be all new and we'd be waiting for Gauss, and real analysis would be as old as the hills, and people would be asking "why is [insert arbitrary thing] so important in real analysis"?
[PS (1) yeah I know, I was being facetious at the end, and (2) yeah I know, my list at the top is woefully incomplete]
A: May I suggest that we don't have to consider cohomology to see the influence
of topology on arithmetic? Looking for rational points on curves leads to the
question of which curves have rational parameterization, and Riemann found
that the answer is topological -- the curves of genus zero. One also observes
special behavior on curves of genus 1 (elliptic curves), etc.
A: New lectures by Atiyah on physics-inspired questions in geometry mention some fascinating connections with number theory, e.g. the question about a "quantum analogue of the Weil conjectures"and an "infinite dimensional version" of them.     
