Obviously, I'm biased as I work in finance. FWIW, I get no where near £70k bonus.
For a start, in any bank, on 1/3 of the staff are actual "front office staff" (that is, bankers). The rest is support staff, middle and bank office (IT, operations, catering, PAs, legal, compliance, audit, accountants etc. etc.)
Big bonuses go to those who earn the bank big revenue. One of the heads of depts at my old employer was paid millions in bonuses every year, but he generated EUR6bn in revenue in about three years. If you don't compensate that sort of work and talent, they go elsewhere.
Generally, junior staff don't get much - a junior M&A banker will typically be working 9am to 1/2am every day during the week, and weekends. Starting salary is around £35k. One intern I know in M&A is pretty much eating sandwiches for every meal, as she only gets about 10 minutes for lunch around 4pm. Only after you've done about ten years of that (and not seen friends, family very much, no social life) do you get to the point where you're given a bonus that is multiples of your salary and you start working normal-ish hours. DP also works in an investment bank, not as a banker, and he's on day 7 of working a row - and has been working at least 12 hours a day for the last month.
Banks also sponsor things like art galleries, museums, sports events (e.g. Ryder Cup, World Cup). In my area, we actually end up doing a lot of the work that the government should be doing (around preventing illegal arms trading, bribery, corruption etc.) but don't have the resources or staff to do. The industry also ends up writing a lot of legislation on behalf of the government in my area (as again, the government doesn't have the expertise to do it).
Finally, London is rivalling NY to be the largest financial centre in the world, and is definitely the largest financial centre in Europe. This means that the financial sector (including law firms, accounting firms, consultancy etc. etc.) is bringing in revenue from other countries, as corporations in say Germany will use Deutsche or Commerz bank, but a lot of the grunt work (and the booking of revenues) will go to London.