The idea is not to put malicious code in the network. I doubt Coinbase would try something so silly.
But Coinbase, or any other company who makes money with bitcoin, could manipulate their employers ("core developers") to create codes which would benefit coinbase.
Or give coinbase an advantage over other companies. Or make exchanges more profitable than they are now.
It is hard to tell now what they are planning and how would they achieve any gain, but they certainly could try.
Bitcoin follows BIP, Bitcoin Improvement Proposal.
https://en.bitcoin.it/wiki/BIP_0001
The BIP process begins with a new idea for Bitcoin. Each potential BIP must have a champion -- someone who writes the BIP using the style and format described below, shepherds the discussions in the appropriate forums, and attempts to build community consensus around the idea. The BIP champion (a.k.a. Author) should first attempt to ascertain whether the idea is BIP-able. Posting to the [email protected] mailing list (and maybe the Development & Technical Discussion forum) is the best way to go about this.
The process is public, and transparent. Do you believe Coinbase could co-opt some of the Core developers, and manipulate the community to accept a Coinbase-BIP? Then the BIP system failed, what would be a better process to replace BIP?
Accepting (as in listed on https://github.com/bitcoin/bips/) isn't that hard since there are 10 BIP which aimed to increase block size through hard-fork (BIP 100 - 109), but to make community use or activate it are totally different matter.
I meant "to be considered as acceptable" and be merged, then activated.
For instance a Core developer funded by Coinbase, suddenly authored a proposal for developing sharding in Bitcoin.