A lot of people think Senators' travel expenses are to cover their trips back and forth from their "principal residence" to attend the Senate. Senate travel expense rules are set out in a document called "Senators Travel Policy".
While it has a lot of incredibly bureaucratic language about the "64 point system" of travel credits, it has this interesting definition of what is an allowable "Parliamentary function"
1.5.6 “Parliamentary functions” means duties and activities related to the position of a senator, wherever performed, and includes public and official business and partisan matters, but does not include activities related to:a. the election of a member of the House of Commons during an election under the Canada Elections Act, orb. the private business interests of a senator or a member of a senator’s family or household
Thsi section seems to indicate that Senators have a budget not just to get back and forth to their house (wherever that is), but also to attend partisan events.
However, there is a list of examples of funded vs. unfunded travel expenses in Schedule A". Item 3 (page 28) says "Participation in party activities that are purely partisan matters such as election activities" is not funded. But Item 2 says "Participation in party activities that are related to the work of the Senator" is funded. Item 14 (page 29) says that "Speaking engagements or attendance at fundraising events other than those organized by the Senate." is not funded. Confused yet?
To get around the "no funded travel for partisan purposes" rule, a political party sets the date for a fundraiser, and then an assistant starts to call around the community to see if they would like to have an appearance / event / ribbon cutting / information session with a senator that same day or the next. Who wouldn't be honoured to have a senator read a book to kids at the local library or judge your pie eating contest? Piggybacking a party fundraising event using a non-partisan event as cover for the travel lets you claim the cost back and stick the taxpayer with what should be a party expense.
Of course if asked it is portrayed as "I was going to be in town to judge the pie eating contest anyway and the local riding association had me over to speak at a dinner..."
Free from the time demands of House of Commons duties, senators can be available to be keynote speakers at political fundraisers around Canada. This is especially true of "celebrity" (or what passes as a celebrity in Canada) appointments to the Senate, who are big draws at big buck events. Next time you're scratching your head at a Senate appointment, remember why it's good to have a stable of famous journalists and sports figures appointed by your party to the Senate with a budget and travel allowance to headline at fundraising dinners.
They're worth their weight in gold.