To answer this question In a single statement, you can use HttpContext.Current.Itemsfor very short term storage. By Short term storage means, this data is valid for a single HTTP Request. There are many confusion around regarding storing data in HttpContext.Current.Items and storing data in Session variable. In this blog post I am going to describe what are the different scenarios where we can use HttpContext.Current.Items and what is the exact difference with session variable.
Items collections of HttpContext is and IDictionary key-value collections and that are shared across a single HTTPRequest. Yes, HttpContext.Current.Items valid for a single HTTPRequest. Once after processing, server information is sent back to the browser, the variables that were set in the Items[] collection will lost. Where as for Session Variable, information valid for multiple request as this is user specific. The session variable only expires either on Session Time Out or explicitly clear the values.
Let’s have a quick look how we can store information in in HttpContext.Current.Items
And retrieve it like
As I said, HttpContext.Current.Items, stores data for very limited time period, then when we can use this ? Yes, This is extremely useful when we can want to share content between HttpModule and HTTPHandler.

Because each and every client requests pass through the HTTP Pipeline and HTTP Pipeline consists of HTTP Module and HTTP Handler. So If you are writing one custom HTTP Module by Implementing IHttpModule and you want pass some information from this module to current requested page or any other module you can use the HttpContext.Current.Items to store the data.

Similarly, you use HTTPContext Items collection when you are sharing same information across the different instance based on the user request and that request could be change for different request.
While using Context Items collections you need to keep one things that, Items collections holds the objects, so you need to do proper type casting while retrieving it.
To Summarize, in ASP.NET HttpContext.Current.Items allows us to hold information for an single request. We can use it to store short term information. Storing such kind information extremely helpful to send information across different custom modules or to requested pages. You have to make sure that, the data you are using in HttpContext.Current.Items is only valid for that current request and data should be flashed out automatically when request sent to browser for any new request you have to store the data again. Where as session variable is valid for every request unless session timeout not reached or we are explicitly clear the session.