(non-exhaustive, personally biased [of course] list):

1. to easily share your code in a "out of the box" format
2. to force yourself to abstract the ideas of your analysis
3. to force yourself to document
4. to learn to be a better (i.e. less trial-and-errorish) coder by writing your analysis in function-like statements

1. create folder with the name of the package: MyPackage [or use =devtools::create("MyPackage")=to automatically add a bare-bones description file /in an empty folder/]
2. create in this directory another directory called R
3. put the R-code files into this directory
4. within the MyPackage folder create a file named DESCRIPTION with the following (or similar) content:

Package: MyPackage
Version: 0.1
Date: 2017-04-11
Title: My very own package
Description: An R package for something interesting and/or important. etc.
Author:
 Me <me@mymail.eu>,
Maintainer: Me again <me@mymail.eu>
License: GPL-3

1. [/OPTIONAL/ when Roxygen2=is used, see next point]\\ Create a =NAMESPACE file with the following, minimal content:

# Export all names
exportPattern(".")

1. Create the documentation;
   first create in the a subdirectory called man in the package directory. In this folder *.Rd* files for each function are stored in a LaTeX-like format. Since this can be difficult or you might forget to create some files, there is the Roxygen2 package that takes care of creating the *.Rd* files and that forces you the write the documentation of the code directly within the R-code files; besides Roxygen2 creates the NAMESPACE automatically
2. each function argument gets is documented like this:
   #' @param argument_name description of the argument
3. the return value of the function is documented using:
   #' @return something important for A2
4. examples are documented with:
   #' @examples x <- 1
5. The @export line at the end tells Roxygen2 to add the function to the NAMESPACE file:
   #' @export myfunction
6. Finally create the documentation using devtools::document() [in case you are within R and within the package directory] [emacs ess shortcut =C-c C-w d=]
7. build the package: devtools::build()
8. install the package: devtools::install() [emacs ess shortcut =C-c C-w i=]
9. and check: devtools::check() [emacs ess shortcut =C-c C-w c=]

• "R package primer a minimal tutorial" by Karl Broman
• Creating R Packages: A Tutorial by Friedrich Leisch
• Writing R Extensions — the manual from R Core Team

• Creating an R package for Research documentation
• Making Your First R Package