You must have thumbs to be sure your site loads fast. This is best practice in wordpress. For example, if you upload a 5000 x 700 px image and you use that in a page next to other 30 images like that, your site would take forever to load.

We do not recommend deleting any rule as each is used for a certain purpose. If you wish to make changes please follow WordPress resize rules to modify the code. If you wish to remove an image, make sure it is not needed in your theme setup. Help: http://codex.wordpress.org/Function_Reference/add_image_size

Theme resize rules are included in our theme – in wprentals/libs/general-settings.php

They can be changed from Theme Options as well

To regenerate thumbs, you can use this plugin: Force Regenerate Thumbnails plugin – https://wordpress.org/plugins/force-regenerate-thumbnails/ We recommend to use this plugin because allows you to delete all old images size and regenerate the thumbnails for your image attachments.