The question is not why does a partner want to use NetScaler; the question is how broadly can a NetScaler benefit a partner.
For example, partners hosting desktops, applications, web content (including SharePoint), O365/email or offering fully managed IT services, can use NetScaler to load balance servers/services, federate authentication, offload SSL, secure web traffic, proxy ADFS and so on. Any service related service hosted by partners, both privately and/or publically, can benefit from using NetScaler.
NetScaler is available in the Azure and AWS market places and is integrated in to well-known services including Microsoft Intune.
An ADC is integral to optimal and profitable services across the entire the IT industry. The largest and most successful global businesses are using ADCs extensively to secure, optimize and compliment their services.
The Rio Olympics online video content was delivered using NetScaler and many other globally recognised services used in households around the world every day are only possible due to the presence of Citrix NetScaler.