"undefined method `user_signed_in?' when `devise_For` is missing - ruby-on-rails

Question

I wanted to remove the devise routes, as I built custom routes and controllers for my purpose. However, I noticed that when I delete my devise_for :users line in routes and replace it with only...
Routes file
devise_scope :user do
delete 'logout', to: 'devise/sessions#destroy'
end
I am not getting an error:
View:
<% if user_signed_in? %>
Error:
"undefined method `user_signed_in?' for #<#Class:0x00007f984411aab0:0x00007f98441266f8>"
Why would a helper need routes? Which routes are required for it? Can I define only the helpers?
I also plan to use current_user.present? and sign_in #user, but do not know if they will error later, as I can not get passed the undefined Method.
From the Devise Gem: https://github.com/heartcombo/devise/blob/45b831c4ea5a35914037bd27fe88b76d7b3683a4/lib/devise/rails/routes.rb
# Let's say you have an User model configured to use authenticatable,
# confirmable and recoverable modules. After creating this inside your routes:
#
# devise_for :users
#
# This method is going to look inside your User model and create the
# needed routes:
#
# # Session routes for Authenticatable (default)
# new_user_session GET /users/sign_in {controller:"devise/sessions", action:"new"}
# user_session POST /users/sign_in {controller:"devise/sessions", action:"create"}
# destroy_user_session DELETE /users/sign_out {controller:"devise/sessions", action:"destroy"}
#
# # Password routes for Recoverable, if User model has :recoverable configured
# new_user_password GET /users/password/new(.:format) {controller:"devise/passwords", action:"new"}
# edit_user_password GET /users/password/edit(.:format) {controller:"devise/passwords", action:"edit"}
# user_password PUT /users/password(.:format) {controller:"devise/passwords", action:"update"}
# POST /users/password(.:format) {controller:"devise/passwords", action:"create"}
#
# # Confirmation routes for Confirmable, if User model has :confirmable configured
# new_user_confirmation GET /users/confirmation/new(.:format) {controller:"devise/confirmations", action:"new"}
# user_confirmation GET /users/confirmation(.:format) {controller:"devise/confirmations", action:"show"}
# POST /users/confirmation(.:format) {controller:"devise/confirmations", action:"create"}

Answer 1

devise_for is how Devise hooks into the entire framework and sets up its helpers. So skipping entirely is just not how you want to go about it. If you just want to change the name of the paths use the correct option instead:
# availabe path_names are
# :sign_in, :sign_out, :sign_up, :password, :confirmation, :unlock.
devise_for :users, only: :sessions,
path_names: {
sign_out: 'logout'
}
The skip: and only: options can be used to limit the generated routes:
devise_for :users,
only: :sessions,
path_names: {
sign_out: 'logout'
}
devise_for :users,
skip: [:registrations, :passwords, :confirmations],
path_names: {
sign_out: 'logout'
}
If you really want to use the path /user/... instead of /users/...:
devise_for :users, only: :sessions,
path: 'user',
path_names: {
sign_out: 'logout'
}
Not really something I would recommend though.
See ActionDispatch::Routing::Mapper#devise_for.