If it wasn't confusing enough to select the right bid strategy for your campaign(s), you also have to decide if you want to use a Portfolio Bid Strategy. What does even mean?

A Portfolio Bid Strategy is when multiple campaigns in your account work together to achieve the same goal and drive more conversions. With Max Conversions Bid Strategy for example, if you had 3 campaigns using a shared Max Conversions Portfolio Bid Strategy then all 3 campaigns will be learning what converts best based on the data coming in from all 3 campaigns instead of each campaign being in it's own silo learning only from the data coming in from the 1 campaign.

Why would you have separate campaigns in the first place? It can makes sense to have separate campaigns if you're advertising a different product or service. Each product or service would have a different target cost per conversion in this case so by separating them into different campaigns you can more accurately optimize them generate the target cost per conversion for that campaign.

You may also have separate campaigns that are targeting different locations. Sure, you can also set up granular location targeting on a single campaign but by separating it you'll get more granular data about which audience and demographics are converting for a given location.

It's also helpful when you want to have a Shared Budget across all the campaigns. You can't have a Shared Budget without each campaign also using the same Portfolio Bid Strategy.

Shared budgets are helpful to ensure that your budget is being spent no matter what campaign it's being spent in. If you have fixed prices for each campaign there is a chance that one campaign doesn't spend it's budget so the budget is wasted. With a Shared Budget, if one campaign under spends then the remaining budget can be spent in the other campaigns.