Whilst investigating choices of IXPs and browsing through member lists, I realised that Hurricane Electric publicly peered on AMS-IX and other popular IXs. I also came across their peering policy that says they are open to peering with anyone. Whilst I can see the benefit of HE peering with networks on an exchange, I can't quite understand how the other party is mutually benefited. Does peering with a large transit provider at an exchange point offer any benefits for myself? Does it provide the equivelant to (or close to) a transit connection with HE? On a peering agreement you normally don't announce routes announced to you on the IX back up to your own peers, so that would infer it's not the equivelant of transit, so does it provide any increased connectivity?
The peering is for connectivity between HE networks (basically their transit customers) and your networks (your customers) and not for transit between you and the Internet as a whole.
The benefit is they will accept and deliver traffic to/from their transit customers without a complex peering agreement or any fees.
The general idea of peering is that the two operators agree to directly exchange traffic destined to each other's own networks/customers. Peering generally makes sense when doing this will save money and improve connectivity for both parties (typically that would imply that they operate on similar scale).
It sounds like the described scenario may be one where you're not really peers (stricly speaking) but that HE is just very open about who they agree to do peering with. If you're a significantly smaller player and a bigger player is still willing to exchange any traffic at all for free I would think it's more likely to be good for you than for them, provided that the traffic exchanged this way is a non-negligible portion of your total traffic. Often the bigger player may rather see such a situation as an opportunity to have you instead be their customer and refuse to peer on that basis. (You get free access to much more than they do if you peer.)
If you instead were to buy transit from HE I would assume they could give you connectivity to the full Internet if you need that, but you would obviously be paying them for that traffic.
Peering with an internet exchange is probably a worthwhile read as well.