To quote Google, “An attribution model is the rule, or set of rules, that determines how credit for sales and conversions is assigned to touchpoints in conversion paths.”
Translation: the attribution model you use for your PPC advertising dictates which campaigns, ad groups, keywords, ads, etc. get the credit recorded for conversions.
Attribution modelling has been and always will be important because it is a simplification of the complex and numerous interactions your prospects make along the path to a conversion. If you want to optimise marketing you need to understand how much value each channel is being given and ultimately how much it actually deserves.
Attribution modelling is more important than ever as prospect interactions are only getting more complex with more frequent engagement, multiple devices, remarketing campaigns and an increasing number of channels available to the modern advertiser.