They all share files.
Why should anyone use SharePoint instead of SkyDrive or Dropbox?
SharePoint allows you to share files (it's a document library), but it's also
SharePoint is so many things that it is really very hard to categorize but I'd probably call it a 'portal' (a word that is vague enough to cover just about anything), as most places seem to refer to it.
Sharepoint is more than a fileshare. It´s more like a Application Plattform with a set of prebuild features and the Abbility to implement your own logic.
Have a look here for a list of features and compare it to what you can do with dropbox (Btw Skydrive is implented in Sharepoint aka Office 365)
In an (larger) Enterprise settings - SharePoint is viewed as a 'knowledge' management & sharing platform that provides communication, collaboration & "content curation". The latter term is for aggregating content found dispersed across the Enterprise and brought together by search & metadata tools. This post gives a good oversight about it - Sharepoint and Enterprise 2.0: The good, the bad, and the ugly. (it's a bit dated but is still a very relevant reading).
In the above answer - Kirk Broadhurst's point of 'extensible development platform' is very important. SharePoint provides for integration points to external systems like databases, LOB apps like HR & Supply Chain via SharePoint's APIs, BCS, REST et al. Without these, SharePoint and its content will remain a standalone "stove pipe" in the environment.
Microsoft SharePoint is a private intranet site, a data repository, a smart website, a built-in content management system, a development platform, an extranet site, a collection of websites, best-in-class portal software, a document management system, a project management system, a workflow designer, and more. You can collaborate, communicate, gather decision-making reports and data from multiple resources and publish those online, make visually presentable reports, create and view intuitive and real-time dashboards, do customizations, import theme templates, and do more with your SharePoint site.
SharePoint is a web-based collaborative platform that integrates with Microsoft Office. Microsoft SharePoint provides platform that helps your people share, manage and find information working together without wasting time searching and duplicating tasks.Microsoft SharePoint is a collaborative solution, a powerful platform to build and manage Intranets, Extranets and external websites. *SharePoint can easily be configured to meet business needs of companies for business functionality and creative design *Easy content migration and implementation *Security Options available and which can be easily manageable and customized as per requirements *Business portals, Organization portals etc.