[todd-davies]

There are a few ways to think about this. One is deterrence based on cost-benefit analysis, which is essentially a game theoretic way to think about firm behaviour. The logic here would be to fine the firm enough to deter anti-competitive behaviour, as has been mentioned.

Another way to think about it, is to say that we care about safeguarding the process of competition itself. That could include ensuring that competition is fair, ensuring that firms can enter markets, ensuring consumers get to choose which firms to consume from, etc. There's lots of precedent for that in EU competition law [1]. Taking that view, Apple was using its dominant position to restrict the economic freedom of Spotify (and others) and thereby harming the process of competition. Specifically, it limited rival firms from "fully informing iOS users about alternative and cheaper music subscription services" (as per the press release), thus harming competition.

All that to say, if we take the objective of EU competition law as being to prevent large firms from exercising power over smaller firms and to protect the process of competition in a general sense, then these big fines are easier to justify.

[1] https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3166005